Add info about updating the changelog

CONTRIBUTING.md

@@ -2,4 +2,6 @@
 
 * [Coding Standards](coding-standards.md)
 * [How to submit a fix](how-to-submit-a-fix.md)
-* [Getting Pre-Release Castle Packages](prerelease-packages.md)
+* [Getting Pre-Release Castle Packages](prerelease-packages.md)
+
+Every pull request that changes the software (readme and documentation changes excluded) needs to add an entry to the changelog (`CHANGELOG.md` file in the root of each repository). Changes for each release are broken into 3 sections: "Breaking Changes", "Enhancements" and "Bugfixes" (in that order).