Add vcpkg installation instructions (#970)

Summary:
Yoga 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 yoga, ready to be included in their projects.

VCPKG is a C++ library manager that simplifies installation for yoga 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.

Pull Request resolved: https://github.com/facebook/yoga/pull/970

Reviewed By: yungsters

Differential Revision: D40027569

Pulled By: yungsters

fbshipit-source-id: ca9a6aa481c7b46e96c5937fe3cc7b716e464e4d

README.md

@@ -3,6 +3,20 @@
 ## Building
 Yoga builds with [buck](https://buckbuild.com). Make sure you install buck before contributing to Yoga. Yoga's main implementation is in C++, with bindings to supported languages and frameworks. When making changes to Yoga please ensure the changes are also propagated to these bindings when applicable.
 
+Alternatively, you can build and install Yoga using [vcpkg](https://github.com/Microsoft/vcpkg/) dependency manager:
+
+```sh
+git clone https://github.com/Microsoft/vcpkg.git
+cd vcpkg
+./bootstrap-vcpkg.sh
+./vcpkg integrate install
+./vcpkg install yoga
+```
+
+The Yoga 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+If the version is out of date, please [create an issue or pull request](https://github.com/Microsoft/vcpkg) on the vcpkg repository.
+
 ## Testing
 For testing we rely on [gtest](https://github.com/google/googletest) as a submodule. After cloning Yoga run `git submodule init` followed by `git submodule update`.