Add contribution guideline to Gradoop

[ChrizZz110]

Add a CONTRIBUTING.md file to the root describing how to contribute to Gradoop.
It should include infos like:

• Fork the repo to your own profile
• Create a new Branch from develop with a name that starts with the issue number, e.g., #1010_fix_the_bug
• Do as many commits you want in that branch, but please start each commit message with the issue number like [#1010] Added initial classes