Document the vldb (ubik) file format

Briefly document the vldb version 4 file format.  Describe the vl
header, the vl entry, and mh extensions.

Thanks to Marcio Barbosa for an initial draft, comments, and review.

Thanks to Ben Kaduk the prdb.txt, and for an initial review and comments.

Change-Id: If3ca85419027a218b7bb1585f5d5cd4763ad5b26
Reviewed-on: http://gerrit.openafs.org/11564
Reviewed-by: Nathaniel Filardo <nwfilardo@gmail.com>
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>