Add documentation for Websocket API

[shal]

No description provided.

[mod]

We need the list of subscription channels and information between what message goes on public channel and which ones on private channel @yivo @ashanaakh

[mod]

I believe there is other message types.

[shal]

@mod as I see in the websocket protocol source code, here is no private/public channels.
Here:
https://github.com/rubykube/peatio/blob/8537db2b28e35ff71b64a35d494207bd302e6739/app/api/api_v2/websocket_protocol.rb#L47-L48

and here:

https://github.com/rubykube/peatio/blob/8537db2b28e35ff71b64a35d494207bd302e6739/app/api/api_v2/websocket_protocol.rb#L61-L62

I think it is initialized only once on the websocket connection.
Here:
https://github.com/rubykube/peatio/blob/8537db2b28e35ff71b64a35d494207bd302e6739/lib/daemons/websocket_api.rb#L19

Also for me important to mention, that API has on three main types of messages:

• orderbook
• trade
• order

[yivo]

@ashanaakh Can you add some small Ruby script into this doc to demonstrate usage?

[mod]

I don't understand how to use this as API by reading this md file, use more English words please describe make a drawing, do whatever it takes

[shal]

@mod I think there is no use to add drawing here. @yivo I've added ruby code in the bottom, looks much clear for me.