Add vcpkg installation instructions

[FrankXie05]

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

[iaomw]

CMake Error at /home/iaomw/Github/vcpkg/scripts/buildsystems/vcpkg.cmake:834 (_find_package):
  Found package configuration file:

    /home/iaomw/Github/vcpkg/installed/x64-linux/share/unofficial-shaderc/unofficial-shaderc-config.cmake

  but it set unofficial-shaderc_FOUND to FALSE so package
  "unofficial-shaderc" is considered to be NOT FOUND.  Reason given by
  package:

  The following imported targets are referenced, but are missing:
  unofficial::shaderc_util::shaderc_util