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

This commit updates the section on LKMM limitations to no longer say
that SRCU is not modeled, but instead describe how LKMM's modeling of
SRCU departs from the Linux-kernel implementation.

TL;DR:  There is no known valid use case that cares about the Linux
kernel's ability to have partially overlapping SRCU read-side critical
sections.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
Acked-by: Andrea Parri <andrea.parri@amarulasolutions.com>