media: admin-guide: add test-drivers

While test drivers is probably something that normal users
won't bother, it doesn't hurt to add them to the card list.

This way, all drivers, except for the ones under staging,
would be listed there.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/admin-guide/media/cardlist.rst

@@ -703,3 +703,19 @@ xc5000        Xceive XC5000 silicon tuner
 
 	tuner-cardlist
 	frontend-cardlist
+
+Test drivers
+============
+
+In order to test userspace applications, there's a number of virtual
+drivers, with provide test functionality, simulating real hardware
+devices:
+
+=======  ======================================
+Driver   Name
+=======  ======================================
+vicodec  Virtual Codec Driver
+vim2m    Virtual Memory-to-Memory Driver
+vimc     Virtual Media Controller Driver (VIMC)
+vivid    Virtual Video Test Driver
+=======  ======================================