chore: add CONTRIBUTING.md

[tkurki]

Add instructions relating to PR flow and commit
messages. These have been unformalised so far. It
is a bit nasty to ask for changes in PRs when these
have been nowhere to find. Partially copied from
Angular DEVELOPERS.md.

[webmasterkai]

This looks good to me. I might suggest we move the file to /docs so that it automatically shows up on issue creation for new users to review. https://help.github.com/articles/setting-guidelines-for-repository-contributors/

I like the Pull Request and Commit messages section https://github.com/SignalK/specification/blob/master/CONTRIBUTING.md and how it lists the enums.

Might also be worth having an example git commit command to go along withe the others you have there.

[tkurki]

• moved to docs
• added inline commit message guidelines in addition to the link

If you can't find your way to git commit then this doc has no hope - I want to include the stuff that even a bit more experienced people have trouble with.