patman: Add all CC addresses to the cover letter

If we're sending a cover letter make sure to CC everyone that we're
CCing on each of the individual patches.

Signed-off-by: Doug Anderson <dianders@chromium.org>
diff --git a/tools/patman/README b/tools/patman/README
index dc3957c..5b6eba0 100644
--- a/tools/patman/README
+++ b/tools/patman/README
@@ -226,6 +226,9 @@
 will create a patch which is copied to x86, arm, sandbox, mikef, ag and
 afleming.
 
+If you have a cover letter it will get sent to the union of the CC lists of
+all of the other patches.
+
 
 Example Work Flow
 =================