recipes-openharmony/images/README: Add fetch info

As I tried to get the meta-openharmony, some errors have shown up. This MR adds fetch info to the README of building the image

Signed-off-by: Grzegorz Gwóźdź grzegorz.gwozdz@huawei.com

Closes https://gitlab.eclipse.org/eclipse/oniro-core/meta-openharmony/-/issues/45