Document transform options

[robatron]

The configuration docs say that you can set a "text transform style" for the headers (eg comment-divider.mainHeaderTransform) but they don't explain what that means or what the options are. (From transforms.ts it looks like the options are lowercase, uppercase, titlecase, and none, but it'd be nice for them to be listed and explained in the README.)

This is probably a good #firstIssue for someone 😄

Thanks for an awesome extension!