Add default keyboard shortcut to extension description

[Vinnl]

Just a minor quality-of-life improvement that should be easy to implement: I had to look up an article about Pano to find out how to use it, because the description on extensions.gnome.org only mentioned the dependencies I needed to install.

Thanks for Pano btw, it's looking great!

[oae]

Thank you. I will add it to the readme and description of the ego.