Add documentation for the crc muxer.

Signed-off-by: Janne Grunau <janne-ffmpeg@jannau.net>
(cherry picked from commit a4effe432f)

doc/muxers.texi

@@ -18,6 +18,38 @@ enabled muxers.
 
 A description of some of the currently available muxers follows.
 
+@section crc
+
+CRC (Cyclic Redundancy Check) testing format.
+
+This muxer computes and prints the Adler-32 CRC of all the input audio
+and video frames. By default audio frames are converted to signed
+16-bit raw audio and video frames to raw video before computing the
+CRC.
+
+The output of the muxer consists of a single line of the form:
+CRC=0x@var{CRC}, where @var{CRC} is a hexadecimal number 0-padded to
+8 digits containing the CRC for all the decoded input frames.
+
+For example to compute the CRC of the input, and store it in the file
+@file{out.crc}:
+@example
+ffmpeg -i INPUT -f crc out.crc
+@end example
+
+You can print the CRC to stdout with the command:
+@example
+ffmpeg -i INPUT -f crc -
+@end example
+
+You can select the output format of each frame with @file{ffmpeg} by
+specifying the audio and video codec and format. For example to
+compute the CRC of the input audio converted to PCM unsigned 8-bit
+and the input video converted to MPEG-2 video, use the command:
+@example
+ffmpeg -i INPUT -acodec pcm_u8 -vcodec mpeg2video -f crc -
+@end example
+
 @section image2
 
 Image file muxer.