tools/memory-model: doc: Describe the requirement of the litmus-tests directory

It's better that we have some "standard" about which test should be put
in the litmus-tests directory because it helps future contributors
understand whether they should work on litmus-tests in kernel or Paul's
GitHub repo. Therefore explain a little bit on what a "representative"
litmus test is.

Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

Author: fbq

tools/memory-model/README

@@ -195,6 +195,18 @@ litmus-tests
 	are listed in litmus-tests/README.  A great deal more litmus
 	tests are available at https://github.com/paulmckrcu/litmus.
 
+	By "representative", it means the one in the litmus-tests
+	directory is:
+
+		1) simple, the number of threads should be relatively
+		   small and each thread function should be relatively
+		   simple.
+		2) orthogonal, there should be no two litmus tests
+		   describing the same aspect of the memory model.
+		3) textbook, developers can easily copy-paste-modify
+		   the litmus tests to use the patterns on their own
+		   code.
+
 lock.cat
 	Provides a front-end analysis of lock acquisition and release,
 	for example, associating a lock acquisition with the preceding