Create the GitHub Actions guide to create AppImage for python packages

[anki-code]

Hi!

While I creating #18 I thought that it will be great to have the similar algorithm for GitHub Actions that you're already using.

May be create python-appimage-github-actions-example repo with an example of simple PyPi package and with basic GitHub Actions to build AppImage and add it to release asssets. Then packages developers just add it and the world will become a bit portable ;)

Thanks!