Changelog entry for LRU GC (#2122)

* Changelog entry for LRU GC

Author: Greg Soltis

Firestore/CHANGELOG.md

@@ -1,5 +1,14 @@
 # Unreleased
 
+# v0.16.0
+- [changed] Added a garbage collection process to on-disk persistence that
+  removes older documents. This is enabled by default, and the SDK will attempt
+  to periodically clean up older, unused documents once the on-disk cache passes
+  a threshold size (default: 100 MB). This threshold can be configured by
+  setting `FIRFirestoreSettings.cacheSizeBytes`. It must be set to a minimum of
+  1 MB. The garbage collection process can be disabled entirely by setting
+  `FIRFirestoreSettings.cacheSizeBytes` to `kFIRFirestoreCacheSizeUnlimited`.
+
 # v0.15.0
 - [changed] Changed how the SDK handles locally-updated documents while syncing
   those updates with Cloud Firestore servers. This can lead to slight behavior