message: Document 0.17.5 fix for std::chrono::time_point
diff: @@ -128,6 +128,8 @@ is based on [Keep a Changelog](https://keepachangelog.com).
  [0.18.0]).
  - Fix build on GCC 7.2
  - Fix build error in the OpenSSL module under some MSVC configurations
  +- Serializer and deserializer now accept `std::chrono::time_point` for all clock
  + types instead of hard-wiring `std::system_clock`.
  ## [0.17.4] - 2019-02-08
  