Add a changelog entry for Firestore platform logging (#2018)

Author: var-const

firebase-firestore/CHANGELOG.md

@@ -1,10 +1,7 @@
 # Unreleased
-- [feature] Added `Query.whereNotIn()` and `Query.whereNotEqualTo()` query
-  operators. `Query.whereNotIn()` finds documents where a specified field’s
-  value is not in a specified array. `Query.whereNotEqualTo()` finds
-  documents where a specified field's value does not equal the specified value.
-  Neither query operator will match documents where the specified field is not
-  present.
+
+# 21.6.1
+- [changed] Added new internal HTTP headers to the gRPC connection.
 
 # 21.6.0
 - [fixed] Removed a delay that may have prevented Firestore from immediately
@@ -14,6 +11,12 @@
   to the backend immediately after a user signed in.
 - [feature] Cloud Firestore now supports connecting to a local emulator via
  `FirebaseFirestore#useEmulator()`
+- [feature] Added `Query.whereNotIn()` and `Query.whereNotEqualTo()` query
+  operators. `Query.whereNotIn()` finds documents where a specified field’s
+  value is not in a specified array. `Query.whereNotEqualTo()` finds
+  documents where a specified field's value does not equal the specified value.
+  Neither query operator will match documents where the specified field is not
+  present.
 
 # 21.4.3
 - [changed] Firestore now limits the number of concurrent document lookups it