firestore: release v1.2.0

This commit will be tagged firestore/v1.2.0.

Change-Id: I7650658ae1aa21cfc89283d1891bbcb4179a8a12
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/53890
Reviewed-by: Tyler Bui-Palsulich <tbp@google.com>
diff --git a/firestore/CHANGES.md b/firestore/CHANGES.md
index 92381af..7122e52 100644
--- a/firestore/CHANGES.md
+++ b/firestore/CHANGES.md
@@ -1,5 +1,15 @@
 # Changes
 
+## v1.2.0
+
+- Deprecate v1beta1 client.
+- Fix serverTimestamp docs.
+- Add missing operators to query docs.
+- Make document IDs 20 alpha-numeric characters. Previously, there could be more
+  than 20 non-alphanumeric characters, which broke some users. See
+  https://github.com/googleapis/google-cloud-go/issues/1715.
+- Various updates to autogenerated clients.
+
 ## v1.1.1
 
 - Fix bug in CollectionGroup query validation.