"Contributing" doc

[VirtualDOMinic]

Relates to #138

We currently have some dev instructions in the README.md, but - given that the CoC (#138) is typically included as part of a CONTRIBUTING.md (see Formidable's own Victory and Spectacle repos) - and that we might want to add more contribution instructions (e.g. commit message and branch name conventions), I think we should choose between:

• Adding a CONTRIBUTING.md with contribution instructions
• Actively deciding to stick to having everything in the README.md (perhaps with a contents list at the top to make it nicer to use as the file grows)

Preferences?

[VirtualDOMinic]

See linked PR for my preferred implementation (option 1: adding a CONTRIBUTING.md)