Reference updated Carthage docs and backport CHANGELOG (#2291)

* Carthage cert change
* Backport CHANGELOG updates from cl/230001317

Firestore/CHANGELOG.md

@@ -1,18 +1,26 @@
 # Unreleased
 
 # v1.0.0
-- [changed] The `areTimestampsInSnapshotsEnabled` setting is now enabled by
-  default so timestamp fields read from a FIRDocumentSnapshot will be returned
-  as FIRTimestamp objects instead of NSDate. Any code expecting to receive an
-  NSDate object must be updated.
-- [changed] `Transaction.getDocument()` has been changed to return a non-nil
-  `DocumentSnapshot` with `exists` equal to `false` if the document does not
-  exist (instead of returning a nil DocumentSnapshot). Code that was previously
-  doing `if (snapshot) { ... }` must be changed to
-  `if (snapshot.exists) { ... }`.
+- [changed] **Breaking change:** The `areTimestampsInSnapshotsEnabled` setting
+  is now enabled by default. Timestamp fields that read from a
+  `FIRDocumentSnapshot` will be returned as `FIRTimestamp` objects instead of
+  `NSDate` objects. Update any code that expects to recive a `NSDate` object.
+  See [the reference
+  documentation](https://firebase.google.com/docs/reference/ios/firebasefirestore/api/reference/Classes/FIRFirestoreSettings#/c:objc(cs)FIRFirestoreSettings(py)timestampsInSnapshotsEnabled)
+  for more details.
+- [changed] **Breaking change:** `FIRTransaction.getDocument()` has been changed
+  to return a non-nil `FIRDocumentSnapshot` with `exists` equal to `false` if
+  the document does not exist (instead of returning a nil
+  `FIRDocumentSnapshot`).  Code that includes `if (snapshot) { ... }` must be
+  changed to `if (snapshot.exists) { ... }`.
 - [fixed] Fixed a crash that could happen when the app is shut down after
   a write has been sent to the server but before it has been received on
   a listener (#2237).
+- [changed] Firestore no longer bundles a copy of the gRPC certificates, now
+  that the gRPC-C++ CocoaPod includes them. CocoaPods users should be updated
+  automatically. Carthage users should follow the [updated
+  instructions](https://github.com/firebase/firebase-ios-sdk/blob/master/Carthage.md)
+  to get `gRPCCertificates.bundle` from the correct location.
 
 # v0.16.1
 - [fixed] Offline persistence now properly records schema downgrades. This is a