Document customHeaders.

It was missing.

Author: Miranda

README.md

@@ -185,6 +185,9 @@ Function to process the API result with. Should return an array of entries or an
 #### customParams (Object)
 Object of key value pairs to be send along with the request as query parameters.
 
+#### customHeaders (Object)
+Object of key value pairs with custom headers to be sent along with the request.
+
 
 ## Callback Events
 You can make a callback event via props.