This project contains a blank WebExtension addon, a "white canvas" for your new experiment of extending and remixing the Web.
and now?
https://addons.mozilla.org/en-US/developers/
You can find an overview, API docs, guides and how-tos on MDN:
https://developer.mozilla.org/en-US/Add-ons/WebExtensions
or look at the webextensions-examples git repo on Github as a source of API usage examples:
https://github.com/mdn/webextensions-examples
You should also install the web-ext CLI tool, it is very helpful to run, lint and sign a WebExtension from the command line:
https://developer.mozilla.org/en-US/Add-ons/WebExtensions/Getting_started_with_web-ext
npm (or yarn) install -g web-ext
web-ext run -s /path/to/extension