Document how to build and publish.

README.md

@@ -10,3 +10,14 @@ To install signal-spam CLI, use:
 ## Usage
 
     signal-spam BuyMyDogFood.eml
+
+
+# Releasing
+
+To push a new version of `signal-spam` on PyPI:
+
+- Bump the `__version__` in `signalspam.py`.
+- commit, tag, push commit, push tag.
+- `python -m pip install --upgrade build twine`
+- `python -m build`
+- `python -m twine upload dist/*`