Contributing.md

fixes #847 

Documenting how to contribute to the libgit2sharp repo.

Reference & credit to: https://github.com/Haacked/SeeGit/blob/master/CONTRIBUTING.md
The reason I say that is, even though LibGit2 is a submodule, we are not really expecting contributions to that codebase via a PR to https://github.com/libgit2/libgit2sharp?

I suppose I could mention it is a submodule in git?