doc: Introduction to meta xlator.

... in the form of examples. This is by no means exhaustive.  Meta
xlator is not as well known as it should be for its own good.  This
patch is to get the ball rolling.

Change-Id: I5898c37aa15706e886af68f19caac48c175df90b
BUG: 1144282
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/8410
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>