message: add news item for release
diff: News, Updates, and Announcements
  ================================
  +Feb 20, 2019: Version v1.8.1 now released on GitHub
  +---------------------------------------------------
  +
  +New release version 1.8.1 is now available on the [OSPRay GitHub
  +page](https://github.com/ospray/OSPRay/releases/v1.8.1).
  +
  +
  Feb 5, 2019: Version v1.8.0 now released on GitHub
  ---------------------------------------------------
  