Update README.md

Author: green3g

README.md

@@ -17,4 +17,11 @@ Buttons to set a definition expression on a layer to the selected features. Simi
 
 ## External Viewer Button
 
-A configureable button that allows a user to open an external application using an ID property and a url. The addin is configured to allow the user to modify the external url, layer name, and id property. By default, the addin is configured to allow opening Beacon, a well known parcel viewer using the layer "Parcel Boundaries" and the id property "PARCELID".
+A configureable button that allows a user to open an external application using an ID property and a url.
+
+This button is configurable and the following can be provided using the Pro Options dialog.
+
+ - Application URL: The url to the application. Be sure to include the text {0} which will be replaced with the ID property of the selected feature
+ - Layer ID Field: The field name to query in the selected feature
+ - Layer Name: The name of the layer to query in the map
+