docs: update release procedure

Migrate to GitHub Actions.

Author: weakish

README.md

@@ -49,16 +49,9 @@ nosetests
 ## Release a New Version
 
 1. Edit `changelog` and `setup.py` (`version`).
-2. Commit them and add a new tag. Then publish a new release at GitHub.
-3. Publish the package at PyPI with following commands:
-
-```sh
-python3 -m pip install --user --upgrade setuptools wheel
-rm -rf dist
-python3 setup.py sdist bdist_wheel
-python3 -m pip install --user --upgrade twine
-twine upload dist/*
-```
+2. Commit them and send a pull request.
+3. The maintainer will review and merge the pull request, then create a new release at GitHub web UI.
+4. A new version of the package will be published to PyPI automatically (via GitHub Actions).
 
 ## License