Fill Socket API documentation

[saniales]

I am missing Socket endpoints
they will go on https://github.com/thebotguys/golang-bittrex-api/wiki/Socket-API
Useful information is appreciated

[saniales]

Attaching an useful document from Bittrex slack
WebSocketAPI_MarketTracking.docx

[saniales]

The technology behind bittrex sockets is SIgnalR from ASP.NET core
more info here: https://www.asp.net/signalr
A starting point to implement them in Golang: https://github.com/hweom/signalr (may fork it in this repository, but respect the MIT license)