edk2: Remove Contributions.txt and update Readme.md

* Remove the Contributions.txt file from the root of the edk2 repository
  that contained the TianoCore Contribution Agreement.

* Update Readme.md in the root of the edk2 repository to state that
  BSD+Patent is the preferred license for the EDK II project.

* Move the portions of Contributions.txt to Readme.md that describe
  how to contribute along with the commit message format.

* Add to Readme.md that Signed-off-by means that the contributor
  certifies compliance to the Developer's Certificate of Origin 1.1
  https://developercertificate.org

https://bugzilla.tianocore.org/show_bug.cgi?id=1373

This change is based on the following emails:

  https://lists.01.org/pipermail/edk2-devel/2019-February/036260.html
  https://lists.01.org/pipermail/edk2-devel/2018-October/030385.html

RFCs with detailed process for the license change:

  V3: https://lists.01.org/pipermail/edk2-devel/2019-March/038116.html
  V2: https://lists.01.org/pipermail/edk2-devel/2019-March/037669.html
  V1: https://lists.01.org/pipermail/edk2-devel/2019-March/037500.html

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>

Readme.md

@@ -3,17 +3,20 @@
 A modern, feature-rich, cross-platform firmware development environment
 for the UEFI and PI specifications from www.uefi.org.
 
-Contributions to the EDK II open source project are covered by the
-[TianoCore Contribution Agreement 1.1](Contributions.txt)
-
 The majority of the content in the EDK II open source project uses a
-[BSD 2-Clause License](License.txt).  The EDK II open source project contains
-the following components that are covered by additional licenses:
+[BSD-2-Clause Plus Patent License](License.txt).  The EDK II open source project
+contains the following components that are covered by additional licenses:
 * [AppPkg/Applications/Python/Python-2.7.2/Tools/pybench](AppPkg/Applications/Python/Python-2.7.2/Tools/pybench/LICENSE)
 * [AppPkg/Applications/Python/Python-2.7.2](AppPkg/Applications/Python/Python-2.7.2/LICENSE)
 * [AppPkg/Applications/Python/Python-2.7.10](AppPkg/Applications/Python/Python-2.7.10/LICENSE)
 * [BaseTools/Source/C/BrotliCompress](BaseTools/Source/C/BrotliCompress/LICENSE)
 * [MdeModulePkg/Library/BrotliCustomDecompressLib](MdeModulePkg/Library/BrotliCustomDecompressLib/LICENSE)
+* [BaseTools/Source/C/LzmaCompress](BaseTools/Source/C/LzmaCompress/LZMA-SDK-README.txt)
+* [MdeModulePkg/Library/LzmaCustomDecompressLib](MdeModulePkg/Library/LzmaCustomDecompressLib/LZMA-SDK-README.txt)
+* [IntelFrameworkModulePkg/Library/LzmaCustomDecompressLib/Sdk](IntelFrameworkModulePkg/Library/LzmaCustomDecompressLib/LZMA-SDK-README.txt)
+* [BaseTools/Source/C/VfrCompile/Pccts](BaseTools/Source/C/VfrCompile/Pccts/RIGHTS)
+* [EdkCompatibilityPkg/Other/Maintained/Tools/Pccts](EdkCompatibilityPkg/Other/Maintained/Tools/Pccts/README)
+* [MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma](MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/README)
 * [OvmfPkg](OvmfPkg/License.txt)
 * [CryptoPkg/Library/OpensslLib/openssl](CryptoPkg/Library/OpensslLib/openssl/LICENSE)
 
@@ -31,3 +34,115 @@ are listed in [Maintainers.txt](Maintainers.txt).
 * [UDK2017](https://github.com/tianocore/edk2/releases/tag/vUDK2017)
 * [UDK2018](https://github.com/tianocore/edk2/releases/tag/vUDK2018)
 * [edk2-stable201811](https://github.com/tianocore/edk2/releases/tag/edk2-stable201811)
+
+# Code Contributions
+To make a contribution to a TianoCore project, follow these steps.
+1. Create a change description in the format specified below to
+   use in the source control commit log.
+2. Your commit message must include your `Signed-off-by` signature
+3. Submit your code to the TianoCore project using the process
+   that the project documents on its web page.  If the process is
+   not documented, then submit the code on development email list
+   for the project.
+4. It is preferred that contributions are submitted using the same
+   copyright license as the base project. When that is not possible,
+   then contributions using the following licenses can be accepted:
+   * BSD (2-clause): http://opensource.org/licenses/BSD-2-Clause
+   * BSD (3-clause): http://opensource.org/licenses/BSD-3-Clause
+   * MIT: http://opensource.org/licenses/MIT
+   * Python-2.0: http://opensource.org/licenses/Python-2.0
+   * Zlib: http://opensource.org/licenses/Zlib
+
+   For documentation:
+   * FreeBSD Documentation License
+     https://www.freebsd.org/copyright/freebsd-doc-license.html
+
+   Contributions of code put into the public domain can also be
+   accepted.
+
+   Contributions using other licenses might be accepted, but further
+   review will be required.
+
+# Developer Certificate of Origin
+
+Your change description should use the standard format for a
+commit message, and must include your `Signed-off-by` signature.
+
+In order to keep track of who did what, all patches contributed must
+include a statement that to the best of the contributor's knowledge
+they have the right to contribute it under the specified license.
+
+The test for this is as specified in the [Developer's Certificate of
+Origin (DCO) 1.1](https://developercertificate.org/). The contributor
+certifies compliance by adding a line saying
+
+  Signed-off-by: Developer Name <developer@example.org>
+
+where `Developer Name` is the contributor's real name, and the email
+address is one the developer is reachable through at the time of
+contributing.
+
+```
+Developer's Certificate of Origin 1.1
+
+By making a contribution to this project, I certify that:
+
+(a) The contribution was created in whole or in part by me and I
+    have the right to submit it under the open source license
+    indicated in the file; or
+
+(b) The contribution is based upon previous work that, to the best
+    of my knowledge, is covered under an appropriate open source
+    license and I have the right under that license to submit that
+    work with modifications, whether created in whole or in part
+    by me, under the same open source license (unless I am
+    permitted to submit under a different license), as indicated
+    in the file; or
+
+(c) The contribution was provided directly to me by some other
+    person who certified (a), (b) or (c) and I have not modified
+    it.
+
+(d) I understand and agree that this project and the contribution
+    are public and that a record of the contribution (including all
+    personal information I submit with it, including my sign-off) is
+    maintained indefinitely and may be redistributed consistent with
+    this project or the open source license(s) involved.
+```
+
+# Sample Change Description / Commit Message
+
+```
+From: Contributor Name <contributor@example.com>
+Subject: [Repository/Branch PATCH] Pkg-Module: Brief-single-line-summary
+
+Full-commit-message
+
+Signed-off-by: Contributor Name <contributor@example.com>
+```
+
+## Notes for sample patch email
+
+* The first line of commit message is taken from the email's subject
+  line following `[Repository/Branch PATCH]`. The remaining portion of the
+  commit message is the email's content.
+* `git format-patch` is one way to create this format
+
+## Definitions for sample patch email
+
+* `Repository` is the identifier of the repository the patch applies.
+  This identifier should only be provided for repositories other than
+  `edk2`. For example `edk2-BuildSpecification` or `staging`.
+* `Branch` is the identifier of the branch the patch applies. This
+  identifier should only be provided for branches other than `edk2/master`.
+  For example `edk2/UDK2015`, `edk2-BuildSpecification/release/1.27`, or
+  `staging/edk2-test`.
+* `Module` is a short identifier for the affected code or documentation. For
+  example `MdePkg`, `MdeModulePkg/UsbBusDxe`, `Introduction`, or
+  `EDK II INF File Format`.
+* `Brief-single-line-summary` is a short summary of the change.
+* The entire first line should be less than ~70 characters.
+* `Full-commit-message` a verbose multiple line comment describing
+  the change.  Each line should be less than ~70 characters.
+* `Signed-off-by` is the contributor's signature identifying them
+  by their real/legal name and their email address.