Add info fetching openharmony_standard

While bitbake openharmony-standard-image,runninig task openharmony_standard do_fetch, various errors may show up, like:

error: RPC failed; curl 56 GnuTLS recv error (-54): Error in the pull function.

It happens because the repo is giant and unstable connection may prevent the action from being successful.

Solution is probably here. However, sometimes rerunning build is enough (even three times).

We can add information about that to README