update changelog for preconditions (#637)

Author: Brian Chen

firebase-firestore/CHANGELOG.md

@@ -1,4 +1,8 @@
 # Unreleased
+- [changed] Transactions are now more flexible. Some sequences of operations
+  that were previously incorrectly disallowed are now allowed. For example,
+  after reading a document that doesn't exist, you can now set it multiple
+  times successfully in a transaction.
 
 # 20.2.0
 - [fixed] Fixed an issue where query results were temporarily missing documents