Documentation/litmus-tests: Merge atomic's README into top-level one

Where Documentation/litmus-tests/README lists RCU litmus tests,
Documentation/litmus-tests/atomic/README lists atomic litmus tests.
For symmetry, merge the latter into former, with some context
adjustment in the introduction.

Acked-by: Andrea Parri <parri.andrea@gmail.com>
Acked-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Acked-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Documentation/litmus-tests/README

@@ -2,6 +2,25 @@
 LITMUS TESTS
 ============
 
+Each subdirectory contains litmus tests that are typical to describe the
+semantics of respective kernel APIs.
+For more information about how to "run" a litmus test or how to generate
+a kernel test module based on a litmus test, please see
+tools/memory-model/README.
+
+
+atomic (/atomic derectory)
+--------------------------
+
+Atomic-RMW+mb__after_atomic-is-stronger-than-acquire.litmus
+    Test that an atomic RMW followed by a smp_mb__after_atomic() is
+    stronger than a normal acquire: both the read and write parts of
+    the RMW are ordered before the subsequential memory accesses.
+
+Atomic-RMW-ops-are-atomic-WRT-atomic_set.litmus
+    Test that atomic_set() cannot break the atomicity of atomic RMWs.
+
+
 RCU (/rcu directory)
 --------------------