Process tweaks

[ermo]

The overarching goal is to make the process easy enough that it's done as part of the sync process, since the work we do here will translate into ensuring that we have screenshots for apps in both Solus SC, GNOME Software and KDE Discover.

Therefore, this is considered a SHOULD have item in the short term and a MUST have item in the longer term.

Hence, it needs to be documented and automated to the point where it's a cinch.

Joey will be driving the process, but will need help in terms of QA and writing up the docs and documenting the scripts so anyone can study them and learn why we do it like we do.

README changes

The README should be split into three major parts:

• shared prep and setup
• diverging instructions for doing the actual processing locally XOR on the package repo server.
• converging instructions once the processed data has been uploaded to the package repo server.

Shared Server directory

We will want to have a shared server directory for doing the actual processing of the appstream data, so we don't have a bunch of almost-idential copies floating around in people's $HOME dirs.