message: doc: update PyZMQ installation instructions, ZeroMQ link
diff: # Block and Transaction Broadcasting with ZeroMQ
  -[ZeroMQ](http://zeromq.org/) is a lightweight wrapper around TCP
  +[ZeroMQ](https://zeromq.org/) is a lightweight wrapper around TCP
  connections, inter-process communication, and shared-memory,
  providing various message-oriented semantics such as publish/subscribe,
  request/reply, and push/pull.
  @@ -39,8 +39,9 @@ For version information, see [dependencies.md](dependencies.md).
  Typically, it is packaged by distributions as something like
  *libzmq3-dev*. The C++ wrapper for ZeroMQ is *not* needed.
  -In order to run the example Python client scripts in contrib/ one must
  -also install *python3-zmq*, though this is not necessary for daemon
  +In order to run the example Python client scripts in the `contrib/zmq/`
  +directory, one must also install [PyZMQ](https://github.com/zeromq/pyzmq)
  +(generally with `pip install pyzmq`), though this is not necessary for daemon
  operation.
  ## Enabling
  