add mailing list to README (#2241)
add mailing list to README (#2241)

Signed-off-by: Doug Davis <ff3bde741799c0eff815b83a54eda3fc494df54f@us.ibm.com>
jboyd01/service-catalog,MHBauer/service-catalog,MHBauer/service-catalog,jboyd01/service-catalog,jboyd01/service-catalog,kubernetes-incubator/service-catalog,kubernetes-incubator/service-catalog,kubernetes-incubator/service-catalog,MHBauer/service-catalog,jboyd01/service-catalog,jboyd01/service-catalog,jboyd01/service-catalog,kubernetes-incubator/service-catalog,kubernetes-incubator/service-catalog,MHBauer/service-catalog,kubernetes-sigs/service-catalog,kubernetes-sigs/service-catalog,MHBauer/service-catalog,kubernetes-sigs/service-catalog,kubernetes-sigs/service-catalog,MHBauer/service-catalog,kubernetes-sigs/service-catalog,kubernetes-incubator/service-catalog,kubernetes-sigs/service-catalog