retroarch wiki page

@TravMurav, your photos of running retroarch on bq-paella are quite impressive:

https://wiki.postmarketos.org/wiki/BQ_Aquaris_X5_(bq-paella)#Pictures

The other day I tried retroarch, but the default menu didn't react to touch events (I've read it works through XWayland in Phosh?) and I didn't get very far. Since you apparently figured out how to use it in Phosh, and how to set up that cool looking gameboy skin, it would be terrific if you could create a retroarch wiki page that explains the steps.

I'm also curious about the .desktop entries you have in your launcher, did you create them manually or can they be created automatically?

From there, we could look into streamlining it more, so users just need to install the retroarch package and everything works out of the box.