add info about listening to subcollections across documents

Author: rachelsaunders

firestore-bigquery-export/PREINSTALL.md

@@ -7,7 +7,7 @@ The extension creates and updates a [dataset](https://cloud.google.com/bigquery/
 
 Whenever a document is created, updated, deleted, or imported in the specified collection, this extension sends that update to BigQuery. You can then run queries on this mirrored dataset.
 
-Note that this extension only listens for _document_ changes in the collection, but not changes in any _subcollection_. You can, though, install additional instances of this extension to specifically listen to a subcollection or other collections in your database.
+Note that this extension only listens for _document_ changes in the collection, but not changes in any _subcollection_. You can, though, install additional instances of this extension to specifically listen to a subcollection or other collections in your database. Or if you have the same subcollection across documents in a given collection, you can use `{wildcard}` notation to listen to all those subcollections (for example: `chats/{chatid}/posts`).
 
 ### Additional setup