Update readme about HAProxy sockets and the need for emtpy staples

HAProxy 1.6 does not allow staples to be added over the socket if it didn't already have an .ocsp file for a domain when it started. An ugly fix is to touch the [certificatename].pem.oscp file so there is an empty staple (which HAProxy finds valid). HAProxy will then try to serve it and will accept updates from the socket.