message: Prepare to release alpha.39
diff: @@ -11,7 +11,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
  Unreleased section, uncommenting the header as necessary.
  -->
  -## Unreleased
  +<!--## Unreleased-->
  +
  +## [2.0.0-alpha.39] - 2017-04-26
  * Add a Class feature kind for describing all kinds of classes. This is a superclass of the existing elements and mixins.
  * Mix mixins into mixins. A PolymerElementMixin now has all of the members it inherits other mixins it mixes.
  