Added info about force_update in REST sensor (#4161)

source/_components/sensor.rest.markdown

@@ -89,6 +89,11 @@ json_attributes:
   description: A list of keys to extract values from a JSON dictionary result and then set as sensor attributes.
   reqired: false
   type: list, string
+force_update:
+  description: Sends update events even if the value hasn't changed. Useful if you want to have meaningful value graphs in history.
+  reqired: false
+  type: boolean
+  default: False
 {% endconfiguration %}
 
 <p class='note warning'>