doc-rst: add v4l-drivers to index file

Adds documentation for V4L drivers.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
diff --git a/Documentation/index.rst b/Documentation/index.rst
index b61d8de..31273cc 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -17,6 +17,7 @@
    media/media_uapi
    media/media_drivers
    media/dvb-drivers/index
+   media/v4l-drivers/index
 
 Indices and tables
 ==================