README.md and documentation should better explain relation to boost::variant

Visitors who are not well-versed in the history of boost::variant and std::variant don't understand the rationale for having a "variant2" library:

* Was it created before or after variants became part of the standard?
* Is it a replacement for boost variant? for std::variant?
* If so, what, in a nutshell, is the problem with those?
* If not, why, in a nutshell, create yet another variant class?

The README.md should answer this at least briefly; and the documentation - more thoroughly. Currently that's not the case.