It's not our main branch anyway, and "release" makes it more clear what
it's for. There's an industry-wide trend to move away from terms that
invoke master/slave relationships in order to make members of
marginalized populations slightly less uncomfortable.
* Include information about branching strategy
* Re-organize documentation and include information on testing
* Use gerund form in Contributing headlines
* Fix markdown links and expand developing documentation
* Add pipx link and explain test code structure
* Include pull request process in CONTRIBUTING
* Update CONTRIBUTING with small changes from PR review
* Mention failing tests in CONTRIBUTING
* Add information for developers about issue labels
