cmake (Windows): recommend using Visual Studio's built-in CMake support

cmake (Windows): recommend using Visual Studio's built-in CMake support

It is a lot more convenient to use than having to specify the
configuration in CMake manually (does not matter whether using the
command-line or CMake's GUI).

While at it, recommend using `contrib/buildsystems/out/` as build
directory also in the part that talks about running CMake manually.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>