Update changelog (#9566)

Author: peterfriese

Firestore/Swift/CHANGELOG.md

@@ -5,6 +5,12 @@
 # v8.13.0
 - [added] Added support for explicit typing to `DocumentReference.getDocument(as:)`
   and `DocumentSnapshot.data(as:)` to simplify mapping documents (#9101).
+- [changed] `DocumentSnapshot.data(as:)` will decode the document to the type
+  provided. If you expect that a document might *not exist*, use an optional
+  type (e.g. `Book?.self`) to account for this. See
+  [the documentation](https://firebase.google.com/docs/firestore/query-data/get-data#custom_objects)
+  and this [blog post](https://peterfriese.dev/posts/firestore-codable-the-comprehensive-guide/#mapping-simple-types-using-codable)
+  for an in-depth discussion.
 
 # 8.12.1
 - [added] Added async wrapper for `CollectionReference.addDocument()` and