Add light.group platform docs (#4739)

* Add Light Group platform docs

* Group Light →Light Group

* A picture tells a thousand words

* Update for Light Group default name change.

See https://github.com/home-assistant/home-assistant/pull/12962

source/_components/light.group.markdown

@@ -0,0 +1,48 @@
+---
+layout: page
+title: "Light Group"
+description: "Instructions for how to setup light groups within Home Assistant."
+date: 2018-02-23 11:41
+sidebar: true
+comments: false
+sharing: true
+footer: true
+ha_category: Light
+ha_release: 0.65
+ha_iot_class: "Local Push"
+logo: home-assistant.png
+---
+
+The group light platform lets you combine multiple lights into one entity. All child lights of a light group can still be used as usual, but controlling the state of the grouped light will forward the command to each child light.
+
+To enable this platform in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+light:
+  - platform: group
+    name: Kitchen Lights
+    entities:
+      - light.kitchen_ceiling_lights
+      - light.kitchen_under_cabinet_lights
+      - light.kitchen_spot_lights
+      - light.pendant_lights
+```
+
+{% configuration %}
+  name:
+    description: The name of the light group. Defaults to "Light Group".
+    required: false
+    type: string
+  entities:
+    description: A list of entities to be included in the light group.
+    required: true
+    type: string list
+{% endconfiguration %}
+
+<p class='img'>
+<img src='/images/components/light/group.png'>
+Example of the light group "Kitchen Lights".
+</p>
+
+The supported features of all lights will be added together. For example, if you have one RGB light in a group of otherwise brightness-only lights, the light group will be shown with a color picker.