Added changelog entries for the sync.

CHANGES.txt

@@ -1,3 +1,38 @@
+Unreleased Changes
+
+  Protocol Compiler
+  * The proto compiler no longer requires a .proto filename when it is not
+    generating code.
+
+  C++
+  * Arenas are now unconditionally enabled. cc_enable_arenas no longer has
+    any effect.
+  * Fix a memory corruption bug in reflection when mixing optional and
+    non-optional fields.
+  * Make SpaceUsed() calculation more thorough for map fields.
+  * Add stack overflow protection for text format with unknown field values.
+  * FieldPath::FollowAll() now returns a bool to signal if an out-of-bounds
+    error was encountered.
+  * Performance improvements for Map.
+  * Minor formatting fix when dumping a descriptor to .proto format with
+    DebugString.
+  * UBSAN fix in RepeatedField (#2073).
+  * When running under ASAN, skip a test that makes huge allocations.
+  * Fixed a crash that could happen when creating more than 256 extensions in
+    a single message.
+
+  Java
+  * Bugfix in mergeFrom() when a oneof has multiple message fields.
+
+  Python
+  * Print google.protobuf.NullValue as null instead of "NULL_VALUE" when it is
+    used outside WKT Value/Struct.
+  * Fix bug occurring when attempting to deep copy an enum type in python 3.
+
+  Go:
+  * Update go_package options to reference google.golang.org/protobuf module.
+
+
 2020-07-14 version 3.13.0-rc1 (C++/Java/Python/PHP/Objective-C/C#/Ruby/JavaScript)
 
   C++: