[docs] Refer to 'addons' as 'extensions' in the docs and config

[docs] Slightly improve readme

[docs] Add note about comments being accepted in manifest

[feat] Add external JS script loaded in the service worker + explanations

[docs] Add not about debugging service workers

[feat] Make sizing of popup explicit

[docs] Explain how to fork the repo rather than cloning it

[docs] Improve readme

[docs] Fix readme syntax issue

[docs] Add license

[docs] Improve readme

[feat] Add logo

[docs] Improve readme

[brand] Rename skeleton into starter

[doc] First version of README