Write documentation about naming (#97)

[mechite]

#97 makes a change.
I think we should just note the current state to readme (correct this if I am wrong):

• class, interface, etc parent, with inner @RecordBuilder record:
  -> <inner record>Builder
• record with inner @RecordBuilder record:
  -> <outer record>$<inner record>Builder

It's not configurable AFAIK but I don't care for making it configurable.
But it's good to note down the naming behaviour.

Also that reminds me https://avaje.io does not have anything about this.
^{feel free to assign me this @SentryMan}

[rbygrave]

does not have anything about this.

I think there is a saying along the lines ... "If it isn't documented, it doesn't exist" [at least, I've heard it from a guy at work and now it rolls around in my head] and unfortunately for me its a "work on".