chore: update CONTRIBUTING.md with releasing process (GoogleCloudPlatform#109)

Author: grant

Diff for: CONTRIBUTING.md

@@ -25,4 +25,12 @@ information on using pull requests.
 ## Community Guidelines
 
 This project follows [Google's Open Source Community
-Guidelines](https://opensource.google.com/conduct/).
+Guidelines](https://opensource.google.com/conduct/).
+
+## Releasing
+
+This library is released automatically to Packagist when a
+[GitHub release](https://github.com/GoogleCloudPlatform/functions-framework-php/releases)
+is created. Name the release with the version appropriate for the changes 
+included in that release, following semantic versioning. The package will
+automatically be published to Packagist within a few minutes.