document the gitbranch template

doc/git.mdwn

@@ -65,6 +65,10 @@ think about merging them. This is recommended. :-)
 
 ## branches
 
+In order to refer to a branch in one of the above git repositories, for
+example when submitting a [[patch]], you can use the
+[[templates/gitbranch]] template.
+
 Some of the branches included in the main repository include:
 
 * `gallery` contains the [[todo/Gallery]] plugin. It's not yet merged