message: Add release date for 0.18.3
diff: @@ -5,6 +5,8 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  ## [Unreleased]
  +## [0.18.3] - 2021-05-21
  +
  ### Added
  - The `actor_system_config` now has an additional member called
  @@ -695,7 +697,8 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  - Setting the log level to `quiet` now properly suppresses any log output.
  - Configuring colored terminal output should now print colored output.
  -[Unreleased]: https://github.com/actor-framework/actor-framework/compare/0.18.2...master
  +[Unreleased]: https://github.com/actor-framework/actor-framework/compare/0.18.3...master
  +[0.18.3]: https://github.com/actor-framework/actor-framework/releases/0.18.3
  [0.18.2]: https://github.com/actor-framework/actor-framework/releases/0.18.2
  [0.18.1]: https://github.com/actor-framework/actor-framework/releases/0.18.1
  [0.18.0]: https://github.com/actor-framework/actor-framework/releases/0.18.0
  