Add a Vagrantfile

Vagrant offers a painless way to install and use a defined Linux
development environment. We offer a Vagrantfile to that end for two
reasons:

1) To allow Windows users to gain the full power of Linux' Git

2) To offer users an easy path to verify that the issue they are about
   to report is really a Windows-specific issue; otherwise they would
   need to report it to git@vger.kernel.org instead.

Using it is easy: Download and install https://www.virtualbox.org/, then
download and install https://www.vagrantup.com/ (both are Open Source in
case you desire a more complicated installation), then direct your
command-line window to the Git source directory containing the Vagrantfile
and run the commands:

	vagrant up
	vagrant ssh

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>