Add vcpkg installation instructions

[JonLiu1993]

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

We don't have permission to submit pr to your repository, can you agree with us to submit this pr?

Add the following code to README.md

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### Building nuklear - Using vcpkg

You can download and install nuklear using the [vcpkg](https://github.com/Microsoft/vcpkg) dependency manager:

    git clone https://github.com/Microsoft/vcpkg.git
    cd vcpkg
    ./bootstrap-vcpkg.sh
    ./vcpkg integrate install
    ./vcpkg install nuklear

The nuklear port in vcpkg is kept up to date by Microsoft team members and community contributors. If the version is out of date, please [create an issue or pull request](https://github.com/Microsoft/vcpkg) on the vcpkg repository.
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