Contributing.md

fixes #847 

Documenting how to contribute to the libgit2sharp repo.

Reference & credit to: https://github.com/Haacked/SeeGit/blob/master/CONTRIBUTING.md
For unix derivatives (and people who update the binaries) having libgit2 submodule is important. So you need libgit2 source to do anything related to libgit2 binding.