Added FieldValue.increment() to Changelog
# 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.
