patman: Add a -m option to avoid copying the maintainers

The get_maintainers script is a useful default, but sometimes is copies
too many people, or takes a long time to run.

Add an option to disable it and update the README.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/patman/README b/tools/patman/README
index 5fb508b..e466886 100644
--- a/tools/patman/README
+++ b/tools/patman/README
@@ -27,8 +27,8 @@
 
 in one of your commits, the series will be sent there.
 
-In Linux this will also call get_maintainer.pl on each of your
-patches automatically.
+In Linux and U-Boot this will also call get_maintainer.pl on each of your
+patches automatically (unless you use -m to disable this).
 
 
 How to use this tool