cloud: Add CONTRIBUTING
cloud: Add CONTRIBUTING

Change-Id: I160dc61eea8eadf1706c7d1fa8048f286c30564d

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