New documentation section needed: developer howto's

[hmpf]

Suggested:

• How to develop with docker
• How to develop directly on the command line
• How to triage tickets and PR's (what labels are important!)
• How to use towncrier and make a good changelog message
• What we want in a commit-message #818
• How to do pull-requests
• How to merge
• How to safely test notifications #764
• Move "how to override settings" from site-specific-settings.rst to how-to #802

The "Release checklist" could probably also be in this section.

[johannaengland]

I would argue that "How to use towncrier" is already documented, we only need "How to make a good changelog message"

[hmpf]

Yeah we should move "How to use towncrier" to docs, out of README. But one or two files? Might be best to have

• how to use towncrier to create a new message fragment (or not)
• how to write a good changelog message
• how to (and when) to write a good notes message

.. in one file.

[johannaengland]

Yes, we can split the current towncrier documentation into two parts:
Add the last few details about build to the release procedure documentation
Add the rest to a Creating Pull Requests/Commits docs page

[hmpf]

Another howto needed: split out how to override settings file from site-specific-settings.rst so that that file is only a list of settings.