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

[eyalroz]

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.