Document how to use fallback to override N/A string. Fixes #51

README.md

@@ -26,6 +26,7 @@ resources:
 - `entity` _string_: The thermostat entity id **required**
 - `name` _string|false_: Override the card name, or disable showing a name at all. Default is to use the friendly_name of the thermostat provided
 - `decimals` _number_: Specify number of decimals to use: 1 or 0
+- `fallback` _string_: Specify a text to display if a valid set point can't be determined. Defaults to `N/A`
 - `icon` _string|object_: Show an icon next to the card name. You can also pass an object to specify state-specific icons. Defaults state-specific icons radiator/radiator-disabled/snowflake
   - `idle`: _string_: Use this icon for state idle
   - `heat`: _string_ Use this icon for state heat