cloud: add a notice that the Github repo is a mirror
cloud: add a notice that the Github repo is a mirror

Change-Id: I0dd9aa2b26cf9a7ffdc10e7f38480345da7d71f1

blowmage/gcloud-golang,blowmage/gcloud-golang,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,ptomasroos/gcloud-golang,googleapis/google-cloud-go,ptomasroos/gcloud-golang,michael-grosshans/google-cloud-go,Maksadbek/gcloud-golang,googleapis/google-cloud-go,hsinhoyeh/gcloud-golang,roth1002/gcloud-golang,Maksadbek/gcloud-golang,hsinhoyeh/gcloud-golang,roth1002/gcloud-golang,googleapis/google-cloud-go