please add to readme

[njdro]

I couldn't figure out how to use them for the life of me... please add instructions to choose the scheme:

i.e.

To choose your color scheme use Geany's Menu:
View>Editor>Color Schemes>

[codebrainz]

See geany/geany@1ee35c6

[GoGoris]

I support this, I didn't find this option either. I found it thanks to this issue. I thought that I had to change something in a config file.
(I was only used to the filedefs approach, where I could symlink a theme folder. This is of course a lot nicer and easier approach but it would be nice if it was in the readme.)