Add contributing guidelines and code of conduct

[dcwalk]

We should follow the project guidelines for contributing guidelines and code of conduct signposting for this repo (see them here: https://github.com/datatogether/datatogether)...that would include adding a license!

TODOs to mark as done:

• [x] add link to coc
• [x] add link to contributing guidelines
• [x] add license & license template to repo