[media] vivid.txt: add documentation for the vivid driver

The vivid Virtual Video Test Driver helps testing V4L2 applications
and can emulate V4L2 hardware. Add the documentation for this driver.
Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>