Add docs for waterfurnace component (#4467)

* Add docs for waterfurnace component

* address changes from review

* add further review comments

source/_components/waterfurnace.markdown

@@ -0,0 +1,68 @@
+---
+layout: page
+title: "WaterFurnace"
+description: "Instructions on how to integrate WaterFurnace Geothermal System into Home Assistant."
+date: 2018-01-19 17:00
+sidebar: true
+comments: false
+sharing: true
+footer: true
+logo: waterfurnace.png
+ha_category: Hub
+ha_release: 0.62
+ha_iot_class: "Cloud Polling"
+---
+
+The `waterfurnace` component communicates with the WaterFurnace
+Symphony website's WebSocket to show you many of the sensors in your
+system. While not an official API, this is the same backend the
+Symphony website is based on, and should be reasonably stable.
+
+The sensors provided include:
+ - Thermostat Setpoint
+ - Thermostat Current Temp
+ - Leaving Air Temp
+ - Entering Water Loop Temp
+ - Current Humidity
+ - Current Humidity Setpoint
+ - Total system power (in Watts)
+ - Furnace Mode
+
+To use Waterfurnace in your installation, add the following to your `configuration.yaml` file:
+
+```yaml
+# Example configuration.yaml entry
+waterfurnace:
+  username: you@example.com
+  password: secr3tpassword
+  unit: 0123456789AB
+```
+
+{% configuration %}
+username:
+    description: The email address for your Symphony WaterFurnace account
+    required: true
+    type: string
+password:
+    description: The password for your Symphony WaterFurnace account
+    required: true
+    type: string
+unit:
+    description: The unit serial number for your WaterFurnace
+    required: true
+    type: string
+{% endconfiguration %}
+
+
+#### {% linkable_title Limitations %}
+
+The WebSocket interface used by this module requires active polling,
+otherwise the server side shuts down the connection. By default, this
+polling is happening every 10 seconds. All sensors are updated during
+every polling cycle.
+
+While this is communicating with a thermostat, geothermal systems
+operate most efficiently when setbacks are not used, and the home is
+kept at a constant temperature. It remains useful to collect the data
+from the system to understand it's performance, but a full climate
+interface won't be implemented.