Add changelog entry note in CONTRIBUTING.md

Signed-off-by: Pierre Belloy <[email protected]>

CONTRIBUTING.md

@@ -38,6 +38,7 @@ Please make sure you don’t post any sensitive information while reporting issu
 * [Fork](https://help.github.com/articles/fork-a-repo/) the repo.
 * Code!
 * Make sure to format code with `./gradlew formatCode` command.
+* Add a [changelog entry](changelogs/REAMDE.md) under `changelogs/unreleased`.
 * Push your changes to the topic branch in your fork of the repository.
 * Initiate a [pull request](http://help.github.com/send-pull-requests/)
 * Update the issue, adding a comment including a link to the created pull request