doc: update BUILDING.md

Added a note about Visual Studio 2015 feature which should
be installed for building Node.js source code.

PR-URL: https://github.com/nodejs/node/pull/10669


Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Bartosz Sosnowski <bartosz@janeasystems.com>
Reviewed-By: Myles Borins <myles.borins@gmail.com>