Define overview style for IDL format

[mgalloy]

Overview files in the IDL format should look like:

Overview comments.

DIRECTORIES:
  ./: Description goes here. This must be followed by a blank line.

  collection/: Description goes here. Note that the description can flow
               onto following lines. It is the blank line that delimits
               directories.

[mgalloy]

This should also include the directory overview as well.