Adding statsd mapping (home-assistant#3010)

* Adding statsd mapping

* Keep the default configuration sample minimized

source/_components/statsd.markdown

@@ -28,6 +28,18 @@ Configuration variables:
 - **prefix** (*Optional*): Prefix to use. Defaults to `hass`.
 - **rate** (*Optional*): The sample rate. Defaults to 1.
 - **log_attributes** (*Optional*): Log state and attribute changes. This changes the default stats path.
+- **value_mapping** (*Optional*): Map non-numerical values to numerical ones.
 
-StatsD supports various [backends](https://github.com/etsy/statsd/blob/master/docs/backend.md).
+Full example:
 
+```yaml
+# Example configuration.yaml entry
+statsd:
+  prefix: home
+  rate: 5
+  value_mapping:
+    cooling: 1
+    heating: 10
+```
+
+StatsD supports various [backends](https://github.com/etsy/statsd/blob/master/docs/backend.md).