add notes on releasing

RELEASING.md

@@ -0,0 +1,33 @@
+1. Set a variable to the version number for convenience:
+   ```sh
+   ver=x.y.z
+   ```
+1. Update the changelog:
+   ```sh
+   towncrier --version=$ver
+   ```
+1. Push your changes:
+   ```sh
+   git add -u && git commit -m $ver && git push
+   ```
+1. Sanity-check the
+   [changelog](https://github.com/matrix-org/python-signedjson/blob/master/CHANGELOG.md)
+   and update if need be.
+1. Create a signed tag for the relese:
+   ```sh
+   git tag -s v$ver
+   ```
+   Base the tag message on the changelog.
+1. Push the tag:
+   ```sh
+   git push origin tag v$ver
+   ```
+1. Build and upload to PyPI:
+   ```sh
+   python setup.py sdist
+   twine upload dist/python-signedjson-$ver.tar.gz
+   ```
+1. Create release on GH project page:
+   ```sh
+   xdg-open https://github.com/matrix-org/python-signedjson/releases/edit/v$ver
+   ```