Adds explicit mention of CHANGELOG to CONTRIBUTING file.

Author: Potherca

CONTRIBUTING.md

@@ -4,14 +4,13 @@ Contributions are **welcome** and will be fully **credited**.
 
 We accept contributions via Pull Requests on [Github](https://github.com/potherca/flysystem-github).
 
-
 ## Pull Requests
 
 - **[PSR-2 Coding Standard](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md)** - The easiest way to apply the conventions is to install [PHP Code Sniffer](http://pear.php.net/package/PHP_CodeSniffer).
 
 - **Add tests!** - Your patch won't be accepted if it doesn't have tests.
 
-- **Document any change in behaviour** - Make sure the `README.md` and any other relevant documentation are kept up-to-date.
+- **Document any change in behaviour** - Make sure the `README.md`, `CHANGELOG.md` and any other relevant documentation are kept up-to-date.
 
 - **Consider our release cycle** - We try to follow [SemVer v2.0.0](http://semver.org/). Randomly breaking public APIs is not an option.
 
@@ -21,7 +20,6 @@ We accept contributions via Pull Requests on [Github](https://github.com/potherc
 
 - **Send coherent history** - Make sure each individual commit in your pull request is meaningful. If you had to make multiple intermediate commits while developing, please [squash them](http://www.git-scm.com/book/en/v2/Git-Tools-Rewriting-History#Changing-Multiple-Commit-Messages) before submitting.
 
-
 ## Running Tests
 
 ``` bash