add pypi release guide

Author: TomNicholas

HOW_TO_RELEASE.md

@@ -0,0 +1,13 @@
+1. Make all the code changes, including updating version numbers in `pyproject.toml` and `README.md`
+
+2. Issue the release on github, creating a new tag
+
+3. Fetch all the tags with `git pull`
+
+4. Delete any builds for old versions in `/dist`
+
+5. Build the new distribution using `python setup.py sdist bdist_wheel`
+
+6. Upload to pypi with `twine upload dist/*`
+
+7. Check that the [pypi package page](https://pypi.org/project/cubed-xarray/) shows the expected version.