Added GTT Sensor documentation (#7504)

* Added GTT Sensor documentation

* Update sensor.gtt.markdown

source/_components/sensor.gtt.markdown

@@ -0,0 +1,39 @@
+---
+layout: page
+title: "GTT"
+description: "Instructions on how to integrate timetable data for a GTT stop within Home Assistant."
+date: 2018-11-13 12:09
+sidebar: true
+comments: false
+sharing: true
+footer: true
+ha_category: Transport
+logo: gtt.png
+ha_iot_class: "Cloud Polling"
+ha_release: 0.85
+---
+
+
+The `gtt` sensor will give you the departure time of the next bus at the given stop.
+
+To enable this sensor, add the following lines to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+sensor:
+  - platform: gtt
+    stop: '1080'
+    bus_name: '58B'
+```
+{% configuration %}
+stop:
+  description: The name of the stop.
+  required: true
+  type: string
+bus_name:
+  description: The name of the choosen bus.
+  required: false
+  type: string
+{% endconfiguration %}
+
+The data is coming from the [gtt.to.it](http://www.gtt.to.it/cms/) website.