add steam-adwaita-gradience to the readme

[eylles]

add the script steam-adwaita-gradience to the readme, so that users of adwaita for steam can more easily find it, also as mentioned in #237 it can be used as a starting point for new colorthemes.

may add the option to create new themes with an arbitrary name as thanks to how i wrote the script it should be rather easy.

https://github.com/eylles/steam-adwaita-gradience/blob/ca6cf53fd66c1b00abf69499928785deb8bdf642/build-theme.py#L108-L117