Fill out CONTRIBUTING.md

Created by: jmgate

A the moment, CONTRIBUTING.md just has some placeholder text in it. We need to fill it out such that potential contributors understand what is expected when interacting with Trilinos. I can take a crack at this, but I won't be able to start on it till late next week.

@jwillenbring, @bmpersc, any requests as to content?

@trilinos/framework