locking.txt and glossary.txt have been in LKMM's documentation for
quite a while.

Add them in README's introduction of docs and the list of docs at the
bottom.  Add access-marking.txt in the former as well.
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Acked-by: Andrea Parri <parri.andrea@gmail.com>
Cc: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>