Added more infos about pull requests

Requested that they should contain a single commit and be based on the development branch

Author: Markus Humm

CONTRIBUTING.md

@@ -4,3 +4,8 @@ If you like to contribute then either submit a pull request with your proposed
 changes along with a description about what you like to achieve with the 
 modification/addition you propose or send an e-mail to the person listed as main contact
 in notice.txt
+
+When creating a pull request please follow these rules:
+
+* one commit per pull request
+* base your fork/pull request on the development branch, that's the one with the newest code changes