Add instructions to Mini UI or update README.md

[Xorok]

In the README.md, it says that the mini UI has the "same features as a Full UI, just no widgets", but there are no shortcut instructions listed on the start page of the browser on how to display the options menu and show any of the nice stats seen in the screenshots:

https://github.com/paulrouget/servoshell/raw/master/screenshots/options.png?raw=true
https://github.com/paulrouget/servoshell/raw/master/screenshots/debug.png?raw=true

So either the README.md needs updating to clarify that the mini UI doesn't have all features (like any of the options from the menu) and just basic tab management, or the start page needs to be updated to list the shortcut which opens the menu in mini UI.

[paulrouget]

We should add keyboard shortcuts for these. servo/ports uses F12.