docs: add maintainer entry profile for XFS

Create a new document to list what I think are (within the scope of XFS)
our shared goals and community roles.  Since I will be stepping down
shortly, I feel it's important to write down somewhere all the hats that
I have been wearing for the past six years.

Also, document important extra details about how to contribute to XFS.

Cc: corbet@lwn.net
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Chandan Babu R <chandan.babu@oracle.com>