LU-434 Add comment for $SHARED_DIRECTORY variable

LU-434 Add comment for $SHARED_DIRECTORY variable

To try and avoid confusion, a comment was added to better explain how the
$SHARED_DIRECTORY variable is intended to be used. The explanation was taken
from the Lustre mailing list. Specifically, from this message by Andreas:
http://lists.lustre.org/pipermail/lustre-discuss/2011-June/015682.html

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Change-Id: I594283a5963a0ac83bbe88ce8258e361408ec57a
Reviewed-on: http://review.whamcloud.com/940
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Tested-by: Hudson