.pytool/Readme.md: Add reference to new build instructions

.pytool/Readme.md: Add reference to new build instructions

Adds a reference to the new build instructions on the TianoCore wiki
that currently describe building with containers and Stuart.

Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Gerd Hoffmann <kraxel@redhat.com>