LU-4932 doc: desgin doc for LFSCK

LU-4932 doc: desgin doc for LFSCK

LFSCK design documentation has previously been recorded in an wiki
external to the souce code. This commit adds a first draft of the
design highlights of the LFSCK feature. A new directory has been
created to house this and other future design documents modelled
after the Linux Kernel 'Documentation' directory.

This document includes an introduction, quick usage and features.
In addition there are sections that cover the two phase design of
LFSCK as well as a overview of the file system traversal methods
that design have been implemented. Finally, a further reading
section is included as well as a glossary of Lustre specific
terms.

Change-Id: I598261efd7fc4ac71a32a301b8afe8c60b592ace
Signed-off-by: Richard Henwood <richard.henwood@intel.com>
Reviewed-on: http://review.whamcloud.com/10033
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Ned Bass <bass6@llnl.gov>
Reviewed-by: James Nunez <james.a.nunez@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>