Update changelog

Author: yfakariya

CHANGES.txt

@@ -737,11 +737,11 @@ Relase 1.0.0-beta2 2017-10-29
   * Improve exception message in AOT error of Unity.
   * .NET Standard 1.1/1.3 projects now do not depend on System.Linq.Expressions package.
 
-Release 1.0.0-rc1 T.B.D.
+Release 1.0.0-rc1 2017-12-17
 
   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 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.
+  * Fix exceptions thrown by MessagePackObject.AsBinary()/AsString() reports internal type name. Part of #269.