Added coumentation for MH-Z19 CO2 sensor and Bluetooth LE tracker (#827)

* Added MH-Z19 CO2 sensor documentation

* Minor fixes in MH-Z19 documentation
Added Bluetooth Low-energy tracker documentation

source/_components/device_tracker.bluetooth_le_tracker.markdown

@@ -0,0 +1,35 @@
+---
+layout: page
+title: "Bluetooth Tracker"
+description: "Instructions for integrating bluetooth low-energy tracking within Home Assistant."
+date: 2016-08-24 00:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: bluetooth.png
+ha_category: Presence Detection
+ha_iot_class: "Local Poll"
+ha_release: 0.27
+---
+
+This tracker discovers new devices on boot and in regular intervals and tracks bluetooth low-energy devices periodically based on interval_seconds value. It is not required to pair the devices with each other! 
+Devices discovered are stored with 'BLE_' as the prefix for device mac addresses in `known_devices.yaml`.
+
+<p class='note'>
+Requires PyBluez. If you are on Raspbian, make sure you first install `bluetooth` and `libbluetooth-dev` by running `sudo apt install bluetooth libbluetooth-dev`
+</p>
+
+To use the Bluetooth tracker in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+device_tracker:
+  platform: bluetooth_le_tracker
+```
+
+As some BT LE devices change their MAC address regularly, a new device is only discovered when it has been seen 5 times.
+Some BTLE devices (e.g. fitness trackers) are only visible to the devices that they are paired with. In this case, the BTLE tracker won't see this device.
+
+BTLE tracking requires root privileges.
+
+For additional configuration variables check the [Device tracker page](/components/device_tracker/).