Update README.md

Co-authored-by: Barry Pollard <barrypollard@google.com>

README.md

@@ -428,7 +428,7 @@ For full installation and usage instructions, see Simo's post: [Track Core Web V
 
 When using the [attribution build](#attribution-build), you can send additional data to help you debug _why_ the metric values are they way they are.
 
-This example sends an additional `debug_target` param to Google Analytics, corresponding to the element most associated with each metric.
+This example sends an additional `debug_target` param to Google Analytics, corresponding to the element most associated with each metric. Note this example is for GA4. For Google Analytics prior to GA4 the attribution data should be set in a custom dimension rather in `debug_target` as shown above.
 
 ```js
 import {onCLS, onFID, onLCP} from 'web-vitals/attribution';