Add vcpkg installation instructions

[Cheney-W]

doctest available as a port in vcpkg, a C++ library manager that simplifies installation for doctest and other project dependencies. Documenting the install process here will help users get started by providing a single set of commands to build doctest, ready to be included in their projects.

We also test whether our library ports build in various configurations (dynamic, static) on various platforms (OSX, Linux, Windows: x86, x64) to keep a wide coverage for users.

I'm a maintainer for vcpkg, and here is what the port script looks like. We try to keep the library maintained as close as possible to the original library.