Add contributing guidelines (again?)

This issue has been migrated from mozilla/webmaker-android#2360.

It was originally written by toolness on Mon Jun 29 2015 and had the following description:

It seems like a CONTRIBUTING.md was added in mozilla/webmaker-android#1160, but I see no sign of the file anymore... Where did it go?

For reference, we've got a decent CONTRIBUTING.md on teach. I could use it as a basis for our own contribution guidelines, though I'd need some help with the Android-specific stuff.

On Mon Jun 29 2015, thisandagain commented:

:+1:

On Mon Jun 29 2015, ryanw-se commented:

:+1: Definitely think this would be a good idea, would be good to include contact information for the IRC and forums somewhere in it as well.