Add commented-out theme configurations in initializer

[gazayas]

Addresses #666.

We have more options besides BulletTrain::Themes::Light.color, but I don't think we document them anywhere, and I think it makes sense to just comment them out here in the initializer similar to something you'd see in config/environments/development.rb for example.

# Raises error for missing translations.
# config.i18n.raise_on_missing_translations = true

[olbrich]

I suggest that you also add a comment telling people to replace the logo file with their own logo.