Add London Undergound doc page (#2902)

* Add London tube status doc page

Documentation and example configuration for tube_state sensor.

* Rename and update

Points to updated london_tube component

* Update name

* Update 'ha_release'

source/_components/sensor.london_underground.markdown

@@ -0,0 +1,45 @@
+---
+layout: page
+title: "London Undergound"
+description: "Display the current status of London underground & overground lines within Home Assistant."
+date: 2017-07-30 18:45
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: train.png
+ha_category: Transport
+ha_iot_class: "Cloud Polling"
+ha_release: 0.49
+---
+
+
+The `london_underground` sensor will display the status of London underground lines, as well as the Overground, DLR and Tfl rail.
+
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: london_underground
+    line:
+      - Bakerloo
+      - Central
+      - Circle
+      - District
+      - DLR
+      - Hammersmith & City
+      - Jubilee
+      - London Overground
+      - Metropolitan
+      - Northern
+      - Piccadilly
+      - TfL Rail
+      - Victoria
+      - Waterloo & City
+```
+
+Configuration variables:
+
+- **line** (*Required*): Enter the name of at least one line.
+
+Powered by TfL Open Data [TFL](https://api.tfl.gov.uk/).