increase documentation on hue_activate_scene service. (#1475)

This describes the hue_activate_scene service on the light.hue page,
including the answers to questions I've gotten since this went into
HomeAssistant. Hopefully a good start that can be improved on by
folks.

source/_components/light.hue.markdown

@@ -34,3 +34,56 @@ Configuration variables:
 - **allow_unreachable** (*Optional*):  This will allow unreachable bulbs to report their state correctly. By default *name* from the device is used.
 - **filename** (*Optional*): Make this unique if specifying multiple Hue hubs.
 
+### {% linkable_title Using Hue Scenes in Home Assistant %} ###
+
+The Hue platform has it's own concept of Scenes for setting the colors
+of a group of lights at once. Hue Scenes are very cheap, get created
+by all kinds of apps (as it is the only way to have 2 or more lights
+change at the same time), and are rarely deleted. A typical Hue hub
+might have hundreds of scenes stored in them, many that you've never
+used, almost all very poorly named.
+
+To avoid user interface overload we don't expose Scenes
+directly. Instead there is a
+[light.hue_activate_scene]/(/components/light/#service-lighthue_activate_scene)
+service which can be used by `automation` or `script` components. For
+instance:
+
+```
+script:
+  porch_on:
+    sequence:
+      - service: light.hue_activate_scene
+        data:
+          group_name: "Porch"
+          scene_name: "Porch Orange"
+```
+
+*** Finding Group and Scene Names ***
+
+How do you find these names?
+
+The easiest way to do this is only use the scenes from the 2nd
+generation Hue app. That is organized by Room (Group) and Scene
+Name. Use the values of Room name and Scene name that you see in the
+app. You can test these work on the `dev-service` console of your Home
+Assistant instance.
+
+Alternatively, you can dump all rooms and scene names using this
+[gist](https://gist.github.com/sdague/5479b632e0fce931951c0636c39a9578). This
+does **not** tell you which groups and scenes work together but it's
+sufficient to get values that you can test in the `dev-service` console.
+
+*** Caveats ***
+
+The Hue API doesn't activate Scenes directly, only on a Hue Group
+(typically Rooms, especially if using the 2nd gen app). But Hue Scenes
+don't actually reference their group. So heuristic matching is used.
+
+Neither Group names or Scene names are guarunteed unique in Hue. If
+you are getting non deterministic behavior, adjust your Hue scenes via
+the App to be more identifying.
+
+The Hue hub has limitted spaces for Scenes, and will delete Scenes if
+new ones get created that would overflow that space. The API docs say
+this is based on Least Recently Used.