Add development docs, mostly to document the release process [skip ci]

Author: PeterJCLaw

DEVELOPMENT.md

@@ -0,0 +1,21 @@
+# Development
+
+Dependencies are managed with [poetry](https://python-poetry.org/): `poetry install`
+
+Tests are run with `./runtests`
+
+## Releasing
+
+CI handles releasing to PyPI.
+Releases on GitHub are created manually.
+
+Here's how to do a release:
+
+ - Get all the desired changes into `master`
+ - Wait for CI to pass that
+ - Add a bump commit (see previous "Declare vX.Y.Z" commits; `poetry version` may be useful here)
+ - Push that commit on master
+ - Create a tag of that version number (`git tag v$(poetry version --short)`)
+ - Push the tag (`git push --tags`)
+ - CI will build & deploy that release
+ - Create a Release on GitHub, ideally with a summary of changes