Update CHANGELOG for Firestore v0.10.1 (firebase#768)

Author: wilhuff

Firestore/CHANGELOG.md

@@ -1,7 +1,10 @@
 # Unreleased
+
+# 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
   crash when the client loses its network connection.
+- [fixed] It's now possible to pass a nil completion block to WriteBatch.commit (#745).
 
 # v0.10.0
 - [changed] Removed the includeMetadataChanges property in FIRDocumentListenOptions