message: add a bit of documentation about the denoiser image op, start changelog
diff: Version History
  ---------------
  -### Changes in v2.0.0:
  +### Changes in v2.0.0 (since alpha):
  +
  +- OSPRay now requires Open VKL v0.8.0+, which alleviates the need for
  +- OSPRay now requires Embree v3.7.0, which lets OSPRay represent groups and
  + instances much more efficiently
  +- new log level enums (warnings as errors)
  +- new debug renderer
  +- merged geometry types
  +- C++ wrapper enhancements (STL vector/array support in cpp::Data)
  +- unused parameter warnings
  +- naming conventions are always camel-case
  +- renderer material lists
  +- new examples app to consolidate interactive apps into one
  +- new benchmark suite
  +- overlapping volume support in both path tracer and scivis renderer
  +- MPI device is now a separate repository
  +- various bug fixes
  +
  +### Changes in v2.0.0 (alpha):
  - New major revision of OSPRay brings API breaking improvements over
  v1.x. See `doc/ospray2_porting_guide.md` for a deeper description of
  