DOCS: Update utterances.md (#1021)

docs/interactive/comments/utterances.md

@@ -19,6 +19,8 @@ html:
 
 Note that the `utteranc.es` UI will not show up when you are previewing your book locally, it must be hosted somewhere on the web to function.
 
+You must also activate the GitHub `utterance.es` app which can be found <a href="https://github.com/apps/utterances">here</a>
+
 ## Configure `utteranc.es`
 
 You can pass optional extra configuration for utterances. You may do so by providing `key:val` pairs alongside `repo:` in the configuration. See