adds mention of participant.js in archiving sample walkthrough

Author: aoberoi

sample/Archiving/README.md

@@ -170,7 +170,8 @@ get(new FreeMarkerTemplateView("/participant") {
 ```
 
 Since this view has no further interactivity with buttons, this is all that is needed for a client
-that is participating in an archived session.
+that is participating in an archived session. Once again, much of the functionality is implemented
+in the client, in code that can be found in the `public/js/participant.js` file.
 
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