Add contribution and release process docs

CONTRIBUTING.md

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+# Contributing to the PHP Library for MongoDB
+
+## Initializing the Repository
+
+Developers who would like to contribute to the library will need to clone it and
+initialize the project dependencies with [Composer](https://getcomposer.org/):
+
+```
+$ git clone https://github.com/mongodb/mongo-php-library.git
+$ cd mongo-php-library
+$ composer update
+```
+
+In addition to installing project dependencies, Composer will check that the
+required extension version is installed. Directions for installing the extension
+may be found [here](http://php.net/manual/en/mongodb.installation.php).
+
+Installation directions for Composer may be found in its
+[Getting Started](https://getcomposer.org/doc/00-intro.md) guide.
+
+## Testing
+
+The library's test suite uses [PHPUnit](https://phpunit.de/), which should be
+installed as a development dependency by Composer.
+
+The test suite may be executed with:
+
+```
+$ vendor/bin/phpunit
+```
+
+The `phpunit.xml.dist` file is used as the default configuration file for the
+test suite. In addition to various PHPUnit options, it defines required
+`MONGODB_URI` and `MONGODB_DATABASE` environment variables. You may customize
+this configuration by creating your own `phpunit.xml` file based on the
+`phpunit.xml.dist` file we provide.
+
+### Testing on HHVM
+
+By default, the PHPUnit script relies on the `php` interpreter for your shell
+(i.e. `#!/usr/bin/env php`). You can run the test suite with HHVM like so:
+
+```
+$ hhvm vendor/bin/phpunit
+```
+
+## Releasing
+
+The follow steps outline the release process for a maintenance branch (e.g.
+releasing the `vX.Y` branch as X.Y.Z).
+
+### Ensure PHP version compatibility
+
+Ensure that the library test suite completes on supported versions of PHP and
+HHVM.
+
+### Transition JIRA issues and version
+
+Update the fix version field for all resolved issues with the corresponding ".x"
+fix version.
+
+Update the version's release date and status from the
+[Manage Versions](https://jira.mongodb.org/plugins/servlet/project-config/PHPLIB/versions)
+page.
+
+Transition all resolved issues for this version to the closed state. If changing
+the issues in bulk, be sure to allow email notifications.
+
+### Update version info
+
+The PHP library uses [semantic versioning](http://semver.org/). Do not break
+backwards compatibility in a non-major release or your users will kill you.
+
+A version constant may be added at a later date (see:
+[PHPLIB-131](https://jira.mongodb.org/browse/PHPLIB-131)). For now, there is
+nothing to update.
+
+### Tag release
+
+The maintenance branch's HEAD will be the target for our release tag:
+
+```
+$ git tag -a -m "Release X.Y.Z" X.Y.Z
+```
+
+### Push tags
+
+```
+$ git push --tags
+```
+
+### Publish release notes
+
+The following template should be used for creating GitHub release notes via
+[this form](https://github.com/mongodb/mongo-php-library/releases/new).
+
+```
+The PHP team is happy to announce that version X.Y.Z of our MongoDB PHP library is now available. This library is a high-level abstraction for the PHP 5, PHP 7, and HHVM drivers (i.e. [`mongodb`](http://php.net/mongodb) extension).
+
+**Release Highlights**
+
+<one or more paragraphs describing important changes in this release>
+
+A complete list of resolved issues in this release may be found at:
+$JIRA_URL
+
+**Documentation**
+
+Documentation for this library may be found at:
+https://docs.mongodb.com/php-library/
+
+**Feedback**
+
+If you encounter any bugs or issues with this library, please report them via this form:
+https://jira.mongodb.org/secure/CreateIssue.jspa?pid=12483&issuetype=1
+
+**Installation**
+
+This library may be installed or upgraded with:
+
+    composer require "mongodb/mongodb=^1.0.0"
+
+Installation instructions for the PHP and HHVM driver may be found in the [PHP.net documentation](http://php.net/manual/en/mongodb.installation.php).
+```
+
+The URL for the list of resolved JIRA issues will need to be updated with each
+release. You may obtain the list from
+[this form](https://jira.mongodb.org/secure/ReleaseNote.jspa?projectId=12483).
+
+If commits from community contributors were included in this release, append the
+following section:
+
+```
+**Thanks**
+
+Thanks for our community contributors for this release:
+
+ * [$CONTRIBUTOR_NAME](https://github.com/$GITHUB_USERNAME)
+```
+
+Release announcements should also be sent to the `mongodb-user@googlegroups.com`
+and `mongodb-announce@googlegroups.com` mailing lists.
+
+Consider announcing each release on Twitter. Significant releases should also be
+announced via [@MongoDB](http://twitter.com/mongodb) as well.