Update contributing guide

[replaceafill]

This updates the contributing guide to reflect our current development process better and address some common problems:

• GitHub automatically uses the stable/* branch for new pull requests. This instructs contributors to use the qa/* branch instead which is the focus of our development.
• We usually request new contributors to sign the contributor agreement before engaging in the code review
• We have introduced pre-commit for formatting and linting our code which replaces the need to use different formatting and linting tools like pylint separately. pre-commit is available for developers in the Docker development environment and enabled as a continuous integration job.
• Archivematica dropped Python 2.7 compatibility a couple of versions ago.
• We prefer not to add more tests based on Django's TestCase anymore since their fixture system is less flexible than pytest.
• Maintaining pyvcr fixtures is problematic as well and recent versions have introduced a compatibility problem for us.

The code review guidelines document has also been updated.

[eviau-artefactual]

Thanks for keeping me in the loop - this looks good to me.

[eviau-artefactual]

The one thing I would add is links to any relevant PR, Issue or something else - e.g. I see that we are not linking to VCR.py anymore, but maybe in the future we would like to know why this has been removed.

[replaceafill]

Thank you @eviau-artefactual! That's a great point. Apologies for not being explicit. I've explained a few of the changes in the pull request description.