Readme: Add instructions for registering additional theme directory.

[iandunn]

AFAIK, this is required to make the themes available. It won't be obvious to everyone, though, so some folks might be confused when they don't see the themes appear in wp-admin.

[kjellr]

That's weird — I haven't needed do to this to get the themes working before.

[iandunn]

huh , it's working in my env w/out this now 🤔

i guess i probably got confused at one point and thought that's why things weren't working, when it was actually something else 🤷🏻