updated README to mention link keepalive fix in opendnp3
updated README to mention link keepalive fix in opendnp3

neilstephens/opendatacon,neilstephens/opendatacon,neilstephens/opendatacon,neilstephens/opendatacon,neilstephens/opendatacon,neilstephens/opendatacon,neilstephens/opendatacon