tools/memory-model: Add access-marking.txt to README

Given that access-marking.txt exists, this commit makes it easier to find.
Reported-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/memory-model/Documentation/README

@@ -47,6 +47,10 @@ DESCRIPTION OF FILES
 README
 	This file.
 
+access-marking.txt
+	Guidelines for marking intentionally concurrent accesses to
+	shared memory.
+
 cheatsheet.txt
 	Quick-reference guide to the Linux-kernel memory model.