docs：add more detail about nsqlookupd http api

[ctlove0523]

current description about GET /lookup miss description about response(http status code and http response body),if add more info about response maybe helpful for user（#44 is good）.

GET /lookup

Returns a list of producers for a topic

Request Params:

name	optional	type	location	description
topic	NO	String	Query	the topic to list producers for

Response Params：

name	type	description
channels	List	channels of this topic
producers	List	producer of this topic

Producer

name	type	description
remote_address	String
hostname	String
broadcast_address	String
tcp_port	Int
http_port	Int
version	String

Request example:

curl -i http://localhost:4161/lookup?topic=test

Response example:

HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
X-Nsq-Content-Type: nsq; version=1.0

{
    "channels":[

    ],
    "producers":[
        {
            "remote_address":"127.0.0.1:56121",
            "hostname":"DESKTOP-TJTHRU9",
            "broadcast_address":"DESKTOP-TJTHRU9",
            "tcp_port":4150,
            "http_port":4151,
            "version":"1.2.1"
        }
    ]
}

If we think this helpful,i can make a PR to add more description about nsqlookupd's api