tools/memory-model: Update README for addition of SRCU

		additional call_rcu() process to the site of the

		emulated rcu-barrier().

	e.	Sleepable RCU (SRCU) is not modeled.  It can be

		emulated, but perhaps not simply.

	e.	Although sleepable RCU (SRCU) is now modeled, there

		are some subtle differences between its semantics and

		those in the Linux kernel.  For example, the kernel

		might interpret the following sequence as two partially

		overlapping SRCU read-side critical sections:

			 1  r1 = srcu_read_lock(&my_srcu);

			 2  do_something_1();

			 3  r2 = srcu_read_lock(&my_srcu);

			 4  do_something_2();

			 5  srcu_read_unlock(&my_srcu, r1);

			 6  do_something_3();

			 7  srcu_read_unlock(&my_srcu, r2);

		In contrast, LKMM will interpret this as a nested pair of

		SRCU read-side critical sections, with the outer critical

		section spanning lines 1-7 and the inner critical section

		spanning lines 3-5.

		This difference would be more of a concern had anyone

		identified a reasonable use case for partially overlapping

		SRCU read-side critical sections.  For more information,

		please see: https://paulmck.livejournal.com/40593.html

	f.	Reader-writer locking is not modeled.  It can be

		emulated in litmus tests using atomic read-modify-write