Add DAFS documentation overview for developers

Add DAFS documentation overview for developers

Add some developer documentation for DAFS. This isn't comprehensive or
extremely detailed, but is intended to provide a larger-picture overview
of some of the pieces of DAFS. More detailed docs are intended to exist
in source comments (doxygen and such), and should already be there for
the most part.

Change-Id: I167b52c3bfb9e6b4b7111b3548ca8b1dafe305ea
Reviewed-on: http://gerrit.openafs.org/1376
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>