Mirror repositories for submodules

Hi,

Would a contribution to add mirror repositories as alternate submodule sources be considered for inclusion? Some projects have mirror repositories on other hosting services, and may have bandwidth limits on their primary hosting service. Being able to indicate mirror repositories for where to check for updates and sources for submodules when doing `git clone --recurse-submodules https://my.repo ` or `git submodule update --init --recursive` would be helpful when there is a timeout.

Regards, Benson