Put a link back to the index in every page footer

Signed-off-by: Shawn O. Pearce <sop@google.com>
diff --git a/Documentation/config-sso.txt b/Documentation/config-sso.txt
index 5271aba..274bf2a 100644
--- a/Documentation/config-sso.txt
+++ b/Documentation/config-sso.txt
@@ -205,3 +205,6 @@
 `external_id` field.  For example, if a Siteminder username was "foo"
 then the external_id field would be populated with "gerrit:foo".
 
+GERRIT
+------
+Part of link:index.html[Gerrit Code Review]