Add vcpkg installation instructions

[PhoebeHui]

LuaSocket is available as a port in VCPKG , documenting the install process here will help users get started by providing a single set of commands to build LuaSocket, ready to be included in their projects.

VCPKG is a C++ library manager that simplifies installation for LuaSocket and other project dependencies, 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.

[ewestbrook]

I vote no. There are already many ways to build and install LuaSocket. Let's improve those, if needed, rather than advertise another.

[avently]

@PhoebeHui hello. Looks like there is no way to build luasocket with lua 5.1 using your package manager, am I right? There is only the latest version of Lua available. How to work around this issue?

[PhoebeHui]

@avently, if you want to use lua 5.1, you can follow below steps:

1. Open ./ports/lua/portfile.cmake, update the URLS and FILENAME with lua 5.1
2. ./vcpkg install lua, it would fail since the hash is not correct, you can copy the 'Actual Hash', and replace the hash value.
3. Commnent out PATCHES vs2015-impl-c99.patch since it's only for latest revision.
4. Check the src and headers in ports/lua/CMakeLists.txt file, remove these file doesn't in lua 5.1, and add the missing one.
5. /vcpkg install lua

[alerque]

Trim this down to a minimal mention of the package availability, not a tutorial on how to use vcpkg itself.