[media] v4l: Document raw bayer 4CC codes

Document guidelines how 4CC codes should be named. Only raw bayer is
included currently. Other formats should be documented later on.
Signed-off-by: Sakari Ailus <sakari.ailus@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>

Documentation/video4linux/4CCs.txt

+Guidelines for Linux4Linux pixel format 4CCs
+============================================
+
+Guidelines for Video4Linux 4CC codes defined using v4l2_fourcc() are
+specified in this document. First of the characters defines the nature of
+the pixel format, compression and colour space. The interpretation of the
+other three characters depends on the first one.
+
+Existing 4CCs may not obey these guidelines.
+
+Formats
+=======
+
+Raw bayer
+---------
+
+The following first characters are used by raw bayer formats:
+
+	B: raw bayer, uncompressed
+	b: raw bayer, DPCM compressed
+	a: A-law compressed
+	u: u-law compressed
+
+2nd character: pixel order
+	B: BGGR
+	G: GBRG
+	g: GRBG
+	R: RGGB
+
+3rd character: uncompressed bits-per-pixel 0--9, A--
+
+4th character: compressed bits-per-pixel 0--9, A--