Update CONTRIBUTING guide.

Author: lokielse

CONTRIBUTING.md

@@ -1,10 +1,11 @@
 # Contributing Guidelines
 
 * Fork the project.
+* Switch to **develop** branch.
 * Make your feature addition or bug fix.
 * Add tests for it. This is important so I don't break it in a future version unintentionally.
 * Commit just the modifications, do not mess with the composer.json or CHANGELOG.md files.
 * Ensure your code is nicely formatted in the [PSR-2](https://github.com/php-fig/fig-standards/blob/master/accepted/PSR-2-coding-style-guide.md)
   style and that all tests pass.
-* Send the pull request.
+* Send the pull request **develop** to **develop**.
 * Check that the Travis CI build passed. If not, rinse and repeat.