Update changelog.

Author: yfakariya

CHANGES.txt

@@ -741,4 +741,7 @@ Release 1.0.0-rc1 T.B.D.
 
   BUG FIXES
   * Fix NRE in .NET Standard 1.1/1.3 build (this issue got mixed in beta2).
-  * Fix built-in Guid/BigInteger always output raw type even if PackerCompatibilityOptions.PackBinaryAsRaw is not specified. #270
+  * Fix built-in Guid/BigInteger always output raw type even if PackerCompatibilityOptions.PackBinaryAsRaw is not specified. #270
+  * Fix MessagePackObject.UnderlyingType reports wrong type for ext types. Part of #269.
+    This bug also caused misleading error message for incompatible type conversion.
+  * Fix exceptions thrown by MessagePackObject.AsBinary()/AsString() reports internal type name. Part of #269.