Updated the changelog.

Changelog

@@ -8,6 +8,43 @@ This document contains change notes for bugfix releases in
 the 4.x series, please see :ref:`whatsnew-4.2` for
 an overview of what's new in Celery 4.2.
 
+4.2.1
+=====
+:release-date: 2018-07-18 11:00 AM IST
+:release-by: Omer Katz
+
+- **Result Backend**: Fix deserialization of exceptions that are present in the producer codebase but not in the consumer codebase.
+
+  Contributed by **John Arnold**
+
+- **Message Protocol Compatibility**: Fix error caused by an invalid (None) timelimit value in the message headers when migrating messages from 3.x to 4.x.
+
+  Contributed by **Robert Kopaczewski**
+
+- **Result Backend**: Fix serialization of exception arguments when exception arguments are not JSON serializable by default.
+
+  Contributed by **Tom Booth**
+
+- **Worker**: Fixed multiple issues with rate limited tasks
+
+  Maintain scheduling order.
+  Fix possible scheduling of a :class:`celery.worker.request.Request` with the wrong :class:`kombu.utils.limits.TokenBucket` which could cause tasks' rate limit to behave incorrectly. 
+  Fix possible duplicated execution of tasks that were rate limited or if ETA/Countdown was provided for them.
+
+  Contributed by :github_user:`ideascf`
+
+- **Worker**: Defensively handle invalid timelimit header values in requests.
+
+  Contributed by **Omer Katz**
+
+Documentation fixes:
+
+
+    - **Matt Wiens**
+    - **Seunghun Lee**
+    - **Lewis M. Kabui**
+    - **Prathamesh Salunkhe**
+
 4.2.0
 =====
 :release-date: 2018-06-10 21:30 PM IST