Contributing.md

fixes #847 

Documenting how to contribute to the libgit2sharp repo.

Reference & credit to: https://github.com/Haacked/SeeGit/blob/master/CONTRIBUTING.md
What I meant is that there was a need to have 'git clone --recursive git@github.com:libgit2/libgitsharp.git. Still, that's not fine, since as I said, not all platforms use SSH. Let's leave it at the user's choice.

@hahmed can we have it rephrased above:

'Fork the repository on GitHub, then clone it using your favorite Git client.' - No more, no less, giving too much info on git specifics is going to be wasteful.