Update README.md

README.md

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 # variant2
-A never-valueless implementation of std::variant
+
+A never-valueless C++14 implementation of [std::variant](http://en.cppreference.com/w/cpp/utility/variant). Requires [mp11](https://github.com/pdimov/mp11) and Boost.Config. Intended to be placed into the `libs/variant2` directory of a Boost clone or release, with mp11 in `libs/mp11`.
+
+To avoid the valueless state, this implementation falls back to using double storage unless
+
+* all the contained types are nothrow move constructible, or
+* at least one contained type has a nothrow default constructor.
+
+Supported compilers:
+
+* g++ 5 or later with -std=c++14 or -std=c++1z
+* clang++ 3.5 or later with -std=c++14 or -std=c++1z
+* Visual Studio 2017