Add a changelog entry on the C++ serializer migration (#4188)

Author: var-const

Firestore/CHANGELOG.md

@@ -4,6 +4,12 @@
   is IN a specified array. `whereField(_:arrayContainsAny:)` finds documents
   where a specified field is an array and contains ANY element of a specified
   array.
+- [changed] Firestore SDK now uses Nanopb rather than the Objective-C Protobuf
+  library for parsing protos. This change does not affect visible behavior of
+  the SDK in any way. While we don't anticipate any issues, please [report any
+  issues with network behavior or
+  persistence](https://github.com/firebase/firebase-ios-sdk/issues/new) that you
+  experience.
 
 # v1.6.1
 - [fixed] Fix a race condition that could cause a segmentation fault during