doc: update BUILDING.md

Added note about vcbuild being included as batch to clarify
that it's not needed from msbuild tools or visual studio.

PR-URL: https://github.com/nodejs/node/pull/8704


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ilkka Myller <ilkka.myller@nodefield.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>