Copy rollershutter and garage_door docs to cover

Re: home-assistant/home-assistant#1949

source/_components/cover.mqtt.markdown

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+---
+layout: page
+title: "MQTT Cover"
+description: "Instructions how to integrate MQTT covers into Home Assistant."
+date: 2016-06-28 17:30
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: mqtt.png
+ha_category: Cover
+---
+
+
+The `mqtt` cover platform enables the possibility to control an MQTT
+cover. The device state will be updated only after receiving the a new
+state from `state_topic`. If these messages are published with RETAIN flag, the
+MQTT device will receive an instant state update after subscription and will
+start with correct state. Otherwise, the initial state will be `unknown`.
+
+There is a state attribute that stores the relative position of the device,
+where 0 means the device is `closed` and all other intermediate positions
+means the device is `open`.
+
+When a state topic is not available, the switch will work in optimistic mode.
+In this mode, the switch will immediately change state after every command.
+Otherwise, the switch will wait for state confirmation from device (message
+from `state_topic`).
+
+Optimistic mode can be forced, even if state topic is available. Try to enable
+it, if experiencing incorrect switch operation.
+
+To use your MQTT binary sensor in your installation, add the following to your
+`configuration.yaml` file:
+
+```yaml
+# Example configuration.yml entry
+cover:
+  platform: mqtt
+  state_topic: "home-assistant/cover"
+  command_topic: "home-assistant/cover/set"
+  name: "MQTT Sensor"
+  optimistic: false
+  qos: 0
+  retain: true
+  state_open: "STATE_OPEN"
+  state_closed: "STATE_CLOSED"
+  service_open: "SERVICE_OPEN"
+  service_close: "SERVICE_CLOSE"
+  value_template: '{% raw %}{{ value.x }}{% endraw %}'
+```
+
+Configuration variables:
+
+- **state_topic** (*Required*): The MQTT topic subscribed to receive sensor values.
+- **name** (*Optional*): The name of the sensor. Default is 'MQTT Sensor'. 
+- **state_open** (*Optional*): The payload that represents open state. Default is"STATE_OPEN"
+- **state_closed** (*Optional*): The payload that represents closed state. Default is "STATE_CLOSED"
+- **service_open** (*Optional*):  The payload that represents open state in service mode. Default is"SERVICE_OPEN"
+- **service_close** (*Optional*): The payload that represents closed state in service mode. Default is "SERVICE_CLOSE"
+- **optimistic** (*Optional*): Flag that defines if switch works in optimistic mode. Default is true if no state topic defined, else false.
+- **qos** (*Optional*): The maximum QoS level of the state topic. Default is 0 and will also be used to publishing messages.
+- **retain** (*Optional*): If the published message should have the retain flag on or not.
+- **value_template** (*Optional*): Defines a [template](/topics/templating/) to extract a value from the payload.
+
+For a quick check you can use the commandline tools shipped with `mosquitto` to send MQTT messages. Set the state of your sensor manually:
+
+```bash
+$  mosquitto_pub -h 127.0.0.1 -t home-assistant/cover/set -m "OFF"
+```
+