Link this repo to external documentation and website

[noelmiller]

This repo should be documented as the preferred method for building your own custom image using Universal Blue images.

[noelmiller]

I believe this is done right @KyleGospo @castrojo @bsherman ?

[Venefilyn]

• :heavy_check_mark: https://bazzite.gg/#diy - links directly to this repo
• :x: https://getaurora.dev/
• :question: https://projectbluefin.io/#scene-developers - links to the forums
• :x: https://projectucore.io/