Contributing.md

fixes #847 

Documenting how to contribute to the libgit2sharp repo.

Reference & credit to: https://github.com/Haacked/SeeGit/blob/master/CONTRIBUTING.md
Can you let me know why we need to add below: (I don't understand :smiley: )

> git clone --recursive

Should I simply add that bit of code after line 19 above?