vcpkg should be added to readme.md

[Sebanisu]

https://github.com/boost-ext/ut/discussions/418 is one of the only places that mentions the name of the vcpkg package name bext-ut. Since Conan is in the readme.md i think somewhere around that area vcpkg should be mentioned. Unless it isn't maintained.

https://github.com/boost-ext/ut/blob/f83e15c7f1edb49b4ec20f866eec06940d18a47d/README.md?plain=1#L190-L193