Contributing guide needed

[pilosus]

Not that we've got a volunteer contributor (#36), contributing guide is needed. Things to cover:

• Add CONTRIBUTING.md with the guide (issue-first, ticket claims/assignments, clojure setup, tests & coverage, specs, changelog and authorship)
• Add AUTHORS.md file
• Add pull request checklist (tests & coverage are ok, CHANGELOG.md updated, AUTHORS.md updated, etc)
• Check Django contributing page for inspiration

[pilosus]

Another way to track authors/contributors is to ask to add github profile to the changelog, like pydantic does