Added documentation for mqtt_statestream component (#3302)

* Added documentation for mqtt_statestream component

* Update 'ha_release'

* Update date

* Add period

source/_components/mqtt_statestream.markdown

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+---
+layout: page
+title: "MQTT Statestream"
+description: "Instructions how to setup MQTT Statestream within Home Assistant."
+date: 2017-09-11 08:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: mqtt.png
+ha_category: Other
+ha_release: 0.54
+ha_iot_class: depends
+---
+
+The `mqtt_statestream` component publishes state changes in Home Assistant to individual MQTT topics.
+
+To enable MQTT Statestream in Home Assistant, add the following section to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+mqtt_statestream:
+  base_topic: homeassistant/states
+```
+
+Configuration variables:
+
+- **base_topic** (*Required*): Base topic used to generate the actual topic used to publish.
+
+## Operation
+
+When any Home Assistant entity changes, this component will publish that change to MQTT.
+
+The topic for each entity is different, so you can easily subscribe other systems to just the entities you are interested in.
+The topic will be in the form `base_topic/domain/entity`.
+
+For example, with the example configuration above, if an entity called 'light.master_bedroom_dimmer' is turned on, this component will publish `on` to `homeassistant/states/light/master_bedroom_dimmer`.