Add screenshots/gif to Readme

[rugk]

Easy and simple task and a follow-up of #25 and #59

The screenshots are now there, but the Readme is not populated yet.

• adding appropriate screenshots (only few ones, the add-on listing shows all of them after all)
• add a GIF there (this is the advantage we only have on GitHub as AMO etc. cannot show a GIF/video on their pages)