Add a line what you need to build to see your changes in officecfg

Change-Id: I38f961e153246b29d0959d0b4491b9f7eecbaf8f
Reviewed-on: https://gerrit.libreoffice.org/7803Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>

officecfg/README

 The schema and default settings for the OpenOffice.org configuration database.
 
+If you change change a file in this module, then a make postprocess is needed after make officecfg.
+
 See also:
 [[configmgr]]