Added FieldValue.increment() to Changelog (#257)

Author: schmidt-sebastian

firebase-firestore/CHANGELOG.md

@@ -1,4 +1,7 @@
 # Unreleased
+- [feature] Added `FieldValue.increment()`, which can be used in `update()`
+  and `set(..., SetOptions.merge())` to increment or decrement numeric field
+  values safely without transactions.
 - [fixed] Fixed calculation of SQLite database size on Android 9 Pie devices.
   Previous method could be off by a few MBs on these devices, potentially
   delaying garbage collection.