Add deprecation checklist document for Substrate (#1583)

fixes #182

This PR adds a document with recommendations of how deprecations should
be handled. Initiated within FRAME, this checklist could be extended to
the rest of the repo.

I want to quote here a comment from @kianenigma that summarizes the
spirit of this new document:
> I would see it as a guideline of "what an extensive deprecation
process looks like". As the author of a PR, you should match this
against your "common sense" and see if it is needed or not. Someone else
can nudge you to "hey, this is an important PR, you should go through
the deprecation process".
> 
> For some trivial things, all the steps might be an overkill.

---------

Co-authored-by: Francisco Aguirre <franciscoaguirreperez@gmail.com>

docs/DEPRECATION_CHECKLIST.md

@@ -0,0 +1,79 @@
+# Deprecation Checklist
+
+This deprecation checklist makes sense while we don’t use [SemVer](https://semver.org/).
+After that, this document will most likely change.
+As deprecation and removal of existing code can happen on any release, we need to be mindful that external builders
+could be impacted by the changes we make.
+The deprecation checklist tries to mitigate this impact, while still keeping the developer experience, the DevEx, as
+smooth as possible.
+
+To start a deprecation process, a new issue with the label `T15-deprecation` needs to be created for correct tracking.
+Then these are the actions to take:
+
+## Hard deprecate by adding a warning message
+
+The warning message shall include a removal month and year, which is suggested to be 6 months after the deprecation
+notice is released.
+This means that the code will be removed in a release within that month (or after, but never before). Please use this
+template, doing so makes it easy to search through the code base:
+
+```rust
+#[deprecated(note = "[DEPRECATED] will be removed after [DATE]. [ALTERNATIVE]")]
+```
+`[ALTERNATIVE]` won't always be possible but offer it if it is.
+
+E.g.
+```rust
+#[deprecated(note = "`GenesisConfig` will be removed after December 2023. Use `RuntimeGenesisConfig` instead.")]
+```
+
+Some pieces of code cannot be labeled as deprecated, like [reexports](https://github.com/rust-lang/rust/issues/30827)
+or [dispatchables](https://github.com/paritytech/polkadot-sdk/issues/182#issuecomment-1691684159), for instance.
+In cases like that we can only make a visible enough comment, and make sure that we [announce the deprecation properly](#announce-the-deprecation-and-removal).
+
+## Remove usage of the deprecated code in the code base
+
+Just make sure that we are not using the deprecated code ourselves.
+If you added the deprecation warning from the previous step, this can be done by making sure that warning is not shown
+when building the code.
+
+## Update examples and tutorials
+
+Make sure that the rust docs are updated.
+We also need [https://docs.substrate.io/](https://docs.substrate.io/) to be updated accordingly. The repo behind it is
+[https://github.com/substrate-developer-hub/substrate-docs](https://github.com/substrate-developer-hub/substrate-docs).
+
+## Announce the deprecation and removal
+
+**At minimum they should be noted in the release log.** Please see how to document a PR [here](https://github.com/paritytech/polkadot-sdk/blob/master/docs/CONTRIBUTING.md#documentation).
+There you can give instructions based on the audience and tell them what they need to do to upgrade the code.
+
+Some breaking changes have a bigger impact than others. When the impact is big the release note is not enough, though
+it should still be the primary place for the notice. You can link back to the changelog files in other channels if you
+want to announce it somewhere else.
+Make sure you are as loud as you need to be for the magnitude of the breaking change.
+
+## Removal version is planned
+
+Depending on the removal date indicated in the deprecation warning in the [first step](#hard-deprecate-by-adding-a-warning-message),
+the nature and the importance of the change, it might make sense to coordinate the release with other developers and
+with the Release team.
+
+## Deprecated code is removed
+
+The deprecated code finally gets removed.
+Don’t forget to [announce this accordingly](#announce-the-deprecation-and-removal).
+
+✅ In order to not forget any of these steps, consider using this template in your deprecation issue:
+
+```markdown
+### Tasks
+
+- [ ] Deprecate code by adding a warning message
+- [ ] Remove usage of the deprecated code in the code base
+- [ ] Update examples and tutorials
+- [ ] Announce code deprecation
+- [ ] Plan removal version
+- [ ] Announce code removal
+- [ ] Remove deprecated code
+```