Create CONTRIBUTING.md

Simple file (which github will show for those making PRs) containing guidelines for making PRs.
Feel free to comment if you want/need anything added. I can pull the same thing to FML too if you'd like, Lex.
Update CONTRIBUTING.md

Add link to wiki page on contributing
Merge pull request #1005 from luacs1998/patch-1

Create CONTRIBUTING.md
added ExplosionEvent.Start and ExplosionEvent.Detonate