Add vcpkg installation instructions

[NancyLi1013]

Mimalloc is available as a port in vcpkg, a C++ library manager that simplifies installation for mimalloc and other project dependencies. Documenting the install process here will help users get started by providing a single set of commands to build mimalloc , 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, UWP, ARM) 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.