Update changelog.

Author: yfakariya

CHANGES.txt

@@ -712,7 +712,7 @@ Release 0.9.2 2017-09-26
   BUG FIXES
   * Fix UAP build drop does not exists in nupkg. #186
 
-Release 1.0.0-beta1 T.B.D.
+Release 1.0.0-beta1 2017-10-01
 
   NEW FEATURES
   * .NET Standard 2.0 which supports serializer source code generation on .NET Core. Note that serializer assembly generation is not supported.