Add CONTRIBUTING.md

[RothAndrew]

What/Why

• Add CONTRIBUTING.md, with the primary objective for now being to document the Terratest-based E2E stuff

Fixes #96

[RothAndrew]

/cc @btlghrants - visibility & collaboration on the other documentation work you're doing.

[btlghrants]

This here's a few things I had lined-up to figure out & include in our "how to contribute" stuff. Perhaps you'll get to 'em first?

• We have a PR + 1 reviewer policy.

  • branches or forks?

• We use signed commits (link gpg setup instructions)!

• After review approval, who merges & what kind of merge is the "default": Merge Commit, Squash & merge, or Rebase & merge?

• We like Draft PRs.

[RothAndrew]

@btlghrants I'm ready for you to take a gander at it when you feel like doing so. Still working on it a bit more so not ready to put it out for review but I'm feeling like I've got some good stuff in there now that I can get feedback on.

[RothAndrew]

/test all

[jeff-mccoy]

I think this is a great start. Personally I'm fine with fork or branch, depending on the dev's role within the team. We can always enhance more later, but I think this is great start.