DOCS(protocol): Add network protocol doc file

We have documentation in the mumble-protocol repository and generated website.

The doc file in this repository serves as a simple reference pointing to those.

See also [mumble-www #59](mumble-voip/mumble-www#59)

docs/dev/Network-Protocol.md

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+# Mumble Network Protocol Documentation
+
+The network protocol between client and server is documented on the website [mumble-protocol.readthedocs.io](https://mumble-protocol.readthedocs.io/).
+
+*That documentation is sourced from the [mumble-protocol repository](https://github.com/mumble-voip/mumble-protocol).*