Update CHANGELOG for Firestore v0.10.2 (firebase#856)

Author: wilhuff

Firestore/CHANGELOG.md

@@ -1,5 +1,13 @@
 # Unreleased
 
+# v0.10.2
+- [changed] When you delete a FirebaseApp, the associated Firestore instances
+  are now also deleted (#683).
+- [fixed] Fixed race conditions in streams that could be exposed by rapidly
+  toggling the network from enabled to disabled and back (#772) or encountering
+  a failure from the server (#835).
+- [fixed] Addressed warnings shown by the latest versions of Xcode and CocoaPods.
+
 # v0.10.1
 - [fixed] Fixed a regression in Firebase iOS release 4.8.1 that could in certain
   cases result in an "OnlineState should not affect limbo documents." assertion