NFSD: Add a subsystem policy document

NFSD: Add a subsystem policy document

Steer contributors to NFSD's patchworks instance, list our patch
submission preferences, and more. The new document is based on the
existing netdev and xfs subsystem policy documents.

This is an attempt to add transparency to the process of accepting
contributions to NFSD and getting them merged upstream.

Suggested-by: "Darrick J. Wong" <djwong@kernel.org>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: NeilBrown <neil@brown.name>
[ cel: Hand-edits to address review comments ]
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>