Documentation/litmus-tests: Cite an RCU litmus test

This commit cites a pertinent RCU-related litmus test.

Co-developed-by: Joel Fernandes (Google) <[e-mail]>
Co-developed-by: Akira Yokosawa <[e-mail]>
[Alan: grammar nit]
[ paulmck: Update commit log and title per Akira feedback. ]
Suggested-by: Alan Stern <[e-mail]>
Signed-off-by: Joel Fernandes (Google) <[e-mail]>
Signed-off-by: Akira Yokosawa <[e-mail]>
Signed-off-by: Paul E. McKenney <[e-mail]>

Documentation/litmus-tests/README

···
 RCU (/rcu directory)
 --------------------
 
+MP+onceassign+derefonce.litmus (under tools/memory-model/litmus-tests/)
+    Demonstrates the use of rcu_assign_pointer() and rcu_dereference() to
+    ensure that an RCU reader will not see pre-initialization garbage.
+
 RCU+sync+read.litmus
 RCU+sync+free.litmus
     Both the above litmus tests demonstrate the RCU grace period guarantee