Provide examples to the installation instructions

[r8vnhill]

The installation instructions are too vague.
My suggestion is to add examples on how to install on the most-common browsers (Chrome/Chromium, Firefox and maybe Safari).

Also, I'd move the installation instructions up, but that may be just a thing of personal taste.

[Nyveon]

Instructions added for Opera and Chrome. I wasn't able to install it in Firefox, that remains to be looked into 🤔 (it used to work?).

Until an official release is made, the installation instructions will be at the bottom next to the disclaimer, to make sure people know what they are getting into by installing this.

[ankaph]

Firefox still doesn't support Manifest v3 extensions, but there's a developer preview available in Firefox 101 (instructions here).