MAINTAINERS: Add "R:" designated-reviewers tag

MAINTAINERS: Add "R:" designated-reviewers tag

A ksummit-discuss email thread looked at the difficulty recruiting
and retaining reviewers.  Paul Walmsley also noted the need for patch
submitters to know who the key reviewers are and suggested adding an
"R:" tag to the MAINTAINERS file to record this information on a
per-subsystem basis.  This commit does just that, and a subsequent
commit tags the designated reviewer for the RCU-related subsystems.

http://lists.linuxfoundation.org/pipermail/ksummit-discuss/2014-May/000830.html

Suggested-by: Paul Walmsley <paul@pwsan.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Acked-by: Steven Rostedt <rostedt@goodmis.org>