tools/memory-model: Add required herd7 version to README file

LKMM and the herd7 tool are co-evolving, and out-of-date herd7 tools
produce inaccurate results, often with no obvious error messages.  This
commit therefore adds the required herd7 version to the LKMM README file.

Longer term, it would be good if .cat files could specify the required
version in a manner allowing herd7 to produce clear diagnostics.

Suggested-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: boqun.feng@gmail.com
Cc: dhowells@redhat.com
Cc: j.alglave@ucl.ac.uk
Cc: linux-arch@vger.kernel.org
Cc: luc.maranget@inria.fr
Cc: nborisov@suse.com
Cc: npiggin@gmail.com
Cc: parri.andrea@gmail.com
Cc: stern@rowland.harvard.edu
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/1519169112-20593-9-git-send-email-paulmck@linux.vnet.ibm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>