branching checklist: Say perhaps no Config.mk changes needed

It is only necessary to change Config.mk if it refers to unstable
branches anywhere.  This time, for example, it didn't.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>

docs/process/branching-checklist.txt

@@ -75,6 +75,7 @@ Ensure references to qemu trees in xen.git's Config.mk are updated.
 Check this with
         grep unstable Config.mk 
 which should produce no output.  Replace as necessary.
+(There may well be none.)
 
 Update newly diverging staging (unstable) according to
 release-technician-checklist.txt section re README etc.