V4L/DVB (12847): cx25821: Add README with todo list

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

drivers/staging/cx25821/README

+Todo:
+	- checkpatch.pl cleanups
+	- sparse cleanups
+
+Please send patches to linux-media@vger.kernel.org
+