tools/memory-model/README: Expand dependency of klitmus7

klitmus7 is independent of the memory model but depends on the
build-target kernel release.
It occasionally lost compatibility due to kernel API changes [1, 2, 3].
It was remedied in a backwards-compatible manner respectively [4, 5, 6].

Reflect this fact in README.

[1]: b899a85 ("compiler.h: Remove ACCESS_ONCE()")
[2]: 0bb95f8 ("Makefile: Globally enable VLA warning")
[3]: d56c0d4 ("proc: decouple proc from VFS with "struct proc_ops"")
[4]: https://github.com/herd/herdtools7/commit/e87d7f9287d1
     ("klitmus: Use WRITE_ONCE and READ_ONCE in place of deprecated ACCESS_ONCE")
[5]: https://github.com/herd/herdtools7/commit/a0cbb10d02be
     ("klitmus: Avoid variable length array")
[6]: https://github.com/herd/herdtools7/commit/46b9412d3a58
     ("klitmus: Linux kernel v5.6.x compat")

NOTE: [5] was ahead of herdtools7 7.53, which did not make an
official release.  Code generated by klitmus7 without [5] can still be
built targeting Linux 4.20--5.5 if you don't care VLA warnings.

Acked-by: Andrea Parri <parri.andrea@gmail.com>
Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/memory-model/README

@@ -28,8 +28,34 @@ downloaded separately:
 See "herdtools7/INSTALL.md" for installation instructions.
 
 Note that although these tools usually provide backwards compatibility,
-this is not absolutely guaranteed.  Therefore, if a later version does
-not work, please try using the exact version called out above.
+this is not absolutely guaranteed.
+
+For example, a future version of herd7 might not work with the model
+in this release.  A compatible model will likely be made available in
+a later release of Linux kernel.
+
+If you absolutely need to run the model in this particular release,
+please try using the exact version called out above.
+
+klitmus7 is independent of the model provided here.  It has its own
+dependency on a target kernel release where converted code is built
+and executed.  Any change in kernel APIs essential to klitmus7 will
+necessitate an upgrade of klitmus7.
+
+If you find any compatibility issues in klitmus7, please inform the
+memory model maintainers.
+
+klitmus7 Compatibility Table
+----------------------------
+
+	============  ==========
+	target Linux  herdtools7
+	------------  ----------
+	     -- 4.18  7.48 --
+	4.15 -- 4.19  7.49 --
+	4.20 -- 5.5   7.54 --
+	5.6  --       HEAD
+	============  ==========
 
 
 ==================