Document release process

[mithrandi]

Having this documented only in my head is suboptimal. The process is basically just "run towncrier, tag, build artifacts, upload to pypi" but there's some subtleties here.

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I'm going to do a release soon to try to address the somewhat dire dependency issues that have arisen as a result of the recent automat release; it sure would be nice to know what the steps are

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(I will absolutely just upload a screwed-up release here with no further info :))

[mithrandi]

The process now is:

1. Update the changelog by running towncrier --version <next version number> (eg. 0.9.4), and branch/commit/push/PR/merge this to master.
2. Create/push a tag (either git signed or annotated) from master with the same name as the version to be released (eg. 0.9.4); note that I think rc versions will not work atm.
3. Run python setup.py sdist bdist_wheel && twine upload dist/*

I will try port over the automated-release-via-GA from Axiom but I'm not yet sure how to integrate towncrier.