Means to design chronos+asyncdispatch 3rd party libraries
It is becoming a case that library authors want to support both chronos and asyncdispatch, but currently there's no one way to do it. E.g. an arbitrary library would switch based on `when defined(ChronosAsync)` but other libraries may choose a different flag, which hits usability for the final user, as they'll need to be aware of all of these dependencies and their respective flags. Therefore I suggest to document this flag in chronos README, so that the end user could just add `-d:chronosAsync` and be pretty confident that all dependencies will switch if they can, while library authors should also follow this suggestion and use this flag to switch.