CHANGE: mention develop

Author: mleutenegger

CONTRIBUTING.md

@@ -41,7 +41,7 @@ Follow these steps to contribute code:
 1. Open an issue describing what you want to change (follow one of the previous
     chapters).
 2. Create a fork and implement your changes
-3. Open a pull request
+3. Open a pull request to `develop`
 
 Before submitting the Pull request, make sure to add a description of your
 changes to the changelog.