Create xiaomi_gw.markdown (#3028)

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Initial commit for the new Xiaomi Gateway component

* Update xiaomi_gw.markdown

Updated sid to mac terminology
Modified release version

* Update xiaomi_gw.markdown

* Update xiaomi_gw.markdown

Tweaked configuration variables

source/_components/xiaomi_gw.markdown

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+---
+layout: page
+title: "Xiaomi Gateway”
+description: "Instructions how to integrate your Xiaomi Gateway within Home Assistant."
+date: 2017-07-21 16:34
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: xiaomi.png
+ha_category: Hub
+ha_release: 0.50
+---
+
+The `xiaomi_gw` platform allows you to integrate the following devices into HA
+
+- Temperature and Humidity Sensor (old and new version)
+- Motion Sensor (old and new version)
+- Door and Window Sensor (old and new version)
+- Button (old and new version)
+- Plug aka Socket (ZigBee version, reports power consumed, power load, state and if device in use)
+- Wall Plug (reports power consumed, power load and state)
+- Aqara Wall Switch (Single)
+- Aqara Wall Switch (Double)
+- Aqara Wall Switch LN (Single)
+- Aqara Wall Switch LN (Double)
+- Aqara Wireless Switch (Single)
+- Aqara Wireless Switch (Double)
+- Cube
+- Gas Leak Detector (reports alarm and density)
+- Smoke Detector (reports alarm and density)
+- Gateway (Light, Illumination Sensor, Ringtone play)
+- Intelligent Curtain
+- Battery
+
+
+What's not available?
+
+- Gateway Radio
+- Gateway Button
+- Water Sensor
+- Aqara Air Conditioning Companion
+- Aqara Intelligent Air Conditioner Controller Hub
+- Decoupled mode of the Aqara Wall Switches (Single & Double)
+- Additional alarm events of the Gas and Smoke Detector: Analog alarm, battery fault alarm (smoke detector only), sensitivity fault alarm, I2C communication failure
+
+{% linkable_title Getting started %}
+
+Follow the setup process using your phone and Mi Home app. From here you will be able to retrieve the key from within the app following [this tutorial](https://community.home-assistant.io/t/beta-xiaomi-gateway-integration/8213/1832)
+
+{% linkable_title Configuration %}
+
+One Gateway
+  ```yaml
+ # You can leave mac empty if you only have one gateway
+ xiaomi:
+   gateways:
+     - mac:
+       key: xxxxxxxxxxxxxxxx
+  ```
+
+ Multiple Gateway
+  ```yaml
+ # 12 characters mac can be obtained from the gateway.
+ xiaomi:
+   gateways:
+     - mac: xxxxxxxxxxxx
+       key: xxxxxxxxxxxxxxxx
+     - mac: xxxxxxxxxxxx
+       key: xxxxxxxxxxxxxxxx
+  ```
+
+Configuration variables:
+- **mac** (*Optional*): The MAC of your gateway. Required if you have more than one.
+- **key** (*Optional*): The key of your gateway. Required if you also want to control lights and switches; sensors and binary sensors will still work.
+- **discovery_retry** (*Optional*): Amount of times Home Assitant should try to reconnect to the Xiaomi Gateway. Default is 3.
+- **interface** (*Optional*): Which network interface to use. Default to any.