Contributing.md

fixes #847 

Documenting how to contribute to the libgit2sharp repo.

Reference & credit to: https://github.com/Haacked/SeeGit/blob/master/CONTRIBUTING.md
If code from elsewhere is used, proper credit and a link to the source should exist in the code comments. Then licensing issues can be checked against LibGit2Sharp's very permissive MIT based open source license.