Connection to MPSServer with WebSockets

Dear Strumenta team,

It was discussed in Slack channel that the connection to MPSServer works on following link "ws://localhost:2904/socket". Please add it to the documentation for [MPSServer WebSocket protocol](https://github.com/Strumenta/MPSServer/blob/mps2020.1/documentation/wsprotocol.md).

With best regards,