message: README: adjust boost::optional example
  The "from_json" example for boost::optional is not complete and should also handle the 'none' case.
diff: @@ -563,7 +563,9 @@ namespace nlohmann {
  }
  static void from_json(const json& j, boost::optional<T>& opt) {
  - if (!j.is_null()) {
  + if (j.is_null()) {
  + opt = boost::none;
  + } else {
  opt = j.get<T>(); // same as above, but with
  // adl_serializer<T>::from_json
  }
  