Add vcpkg installation instructions

[NancyLi1013]

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

[chhtz]

THIS MIRROR IS DEPRECATED, please use this: https://gitlab.com/libeigen/eigen.

Besides, the INSTALL file should just describe how to install the version which one has just downloaded, there is no need to point to alternative ways of automatically downloading and installing Eigen (we also don't mention that you can just sudo apt install libeigen3-dev on many Linux distros).

[NancyLi1013]

Hi @chhtz

thanks for your info.