oss developer experience: CONTRIBUTING.md is missing

[ghuntley]

Can the Wire team please put together something like https://github.com/reactiveui/ReactiveUI/blob/develop/CONTRIBUTING.md for folks like myself that documents the basics such as:

• git commit message conventions
• coding rules ("don't use OverloadedStrings, use ... instead")
• submission guidelines (what type of contributions you'll take)
• we require CLA to be signed (here is a copy of the document, if you need pre-approval from employer)

If you place these files in ~/ of the git repository then GitHub will display them when people send in pull-requests.