Add further steps to Development ReadMe

[lewinpauli]

I had no Idea what I should do after building the project by myself to use this awesome extension in gnome 46

like mentioned by "l0sty" on the gnome extension page (https://extensions.gnome.org/extension/5278/pano/),
it would be useful to have a documentation to the further steps

As far as I know you need to Logout from your gnome session

and after that running "gnome-extensions enable pano@elhan.io"

[Totto16]

We certainly could add a better description on how to build the extension yourself and then how to install it, but our main goal is to just publish it on EGO and then it's just a one click install.

It is only due to the third party dependencies and the delay of #278 that the version supporting gnome 46 isn't on EGO yet. Partly also due to my lack of time 😓