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

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