document MQTT in README

README.md

@@ -78,8 +78,8 @@ The HTTP endpoints (shown below) will be fully functional at this point. You sho
 1. `GET /settings`. Gets current settings as JSON.
 1. `PUT /settings`. Patches settings (e.g., doesn't overwrite keys that aren't present). Accepts a JSON blob in the body.
 1. `GET /radio_configs`. Get a list of supported radio configs (aka `device_type`s).
-1. `GET /gateway_traffic/:device_type`. Starts an HTTP long poll. Returns any Milight traffic it hears. Useful if you need to know what your Milight gateway/remote ID is. Since protocols for RGBW/CCT are different, specify one of `rgbw`, `cct`, or `rgb_cct` as `:device_type. Accepts a JSON blob.
-1. `PUT /gateways/:device_id/:device_type/:group_id`. Controls or sends commands to `:group_id` from `:device_id`.
+1. `GET /gateway_traffic/:device_type`. Starts an HTTP long poll. Returns any Milight traffic it hears. Useful if you need to know what your Milight gateway/remote ID is. Since protocols for RGBW/CCT are different, specify one of `rgbw`, `cct`, or `rgb_cct` as `:device_type.
+1. `PUT /gateways/:device_id/:device_type/:group_id`. Controls or sends commands to `:group_id` from `:device_id`. Accepts a JSON blob. The schema is documented below in the _Bulb commands_ section.
 1. `POST /raw_commands/:device_type`. Sends a raw RF packet with radio configs associated with `:device_type`. Example body:
     ```
     {"packet": "01 02 03 04 05 06 07 08 09", "num_repeats": 10}
@@ -127,6 +127,19 @@ $ curl --data-binary '{"command":"set_white"}' -X PUT http://esp8266/gateways/0x
 true%
 ```
 
+## MQTT
+
+To configure your ESP to integrate with MQTT, fill out the following settings:
+
+1. `mqtt_server`- IP or hostname should work. Specify a port with standard syntax (e.g., "mymqttbroker.com:1884").
+1. `mqtt_topic_pattern` - you can control arbitrary configurations of device ID, device type, and group ID with this. More detail is provided below
+1. (optionally) `mqtt_username`
+1. (optionally) `mqtt_password`
+
+`mqtt_topic_pattern` leverages single-level wildcards (documented [here](https://mosquitto.org/man/mqtt-7.html)). For example, specifying `milight/:device_id/:device_type/:group_id` will cause the ESP to subscribe to the topic `milight/+/+/+`. It will then interpret the second, third, and fourth tokens in topics it receives messages on as `:device_id`, `:device_type`, and `:group_id`, respectively.
+
+Messages should be JSON objects using exactly the same schema that the REST gateway uses for the `/gateways/:device_id/:device_type/:group_id` endpoint. Documented above in the _Bulb commands_ section.
+
 ## UDP Gateways
 
 You can add an arbitrary number of UDP gateways through the REST API or through the web UI. Each gateway server listens on a port and responds to the standard set of commands supported by the Milight protocol. This should allow you to use one of these with standard Milight integrations (SmartThings, Home Assistant, OpenHAB, etc.).