drm/doc: Add a few words on validation with IGT

Also provide some pointers for building IGT as some kernel hackers might
not be that familiar with building stuff on Linux distros.

Signed-off-by: Tomeu Vizoso <tomeu.vizoso@collabora.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/1472715695-19812-1-git-send-email-tomeu.vizoso@collabora.com