Document execute permission for commit-msg in Change-Id docs

Bug: issue 1594
Change-Id: I289dc3d8112c8492ec1fce78502f97cd55e75cbe
Signed-off-by: Chad Horohoe <chorohoe@wikimedia.org>
diff --git a/Documentation/user-changeid.txt b/Documentation/user-changeid.txt
index a3015e1..4e87270 100644
--- a/Documentation/user-changeid.txt
+++ b/Documentation/user-changeid.txt
@@ -54,6 +54,10 @@
 
   $ curl -o .git/hooks/commit-msg http://review.example.com/tools/hooks/commit-msg
 
+Then ensure that the execute bit is set on the hook script:
+
+  $ chmod u+x .git/hooks/commit-msg
+
 For more details, see link:cmd-hook-commit-msg.html[commit-msg].
 
 Change Upload