add release docs

Author: blink1073

RELEASE.md

@@ -0,0 +1,17 @@
+This repository uses [`jupyter_releaser`](https://github.com/jupyter-server/jupyter_releaser) for automated releases.
+
+To create a manual release, update the version number in `jupyter_packaging/__version__.py`, then run the following:
+
+```
+git clean -dffx
+python setup.py sdist
+python setup.py bdist_wheel
+export script_version=`python setup.py --version 2>/dev/null`
+git commit -a -m "Release $script_version"
+git tag $script_version
+git push --all
+git push --tags
+pip install twine
+twine check dist/*
+twine upload dist/*
+```