Add our mailing list URL to the README.
Add our mailing list URL to the README.

citrix-openstack-build/pecan,jdandrea/pecan,ryanpetrello/pecan,citrix-openstack-build/pecan,StackStorm/pecan,jdandrea/pecan,ryanpetrello/pecan,jimrollenhagen/pecan,pecan/pecan,jimrollenhagen/pecan,StackStorm/pecan,pecan/pecan