Add ffmpeg y4m / pipeline description to the online docs

[GoogleCodeExporter]

1. What URL does this issue most closely relate to?
http://www.webmproject.org/...

2. Please describe the issue in full.
I've seen John K. write this message more than a few times so adding somewhere 
in the 'Create' section might be useful to link from webm-discuss and elsewhere.

"
You need to export from ffmpeg to y4m. This can be done with a
 pipeline to avoid storing the raw data to disk, for example:

$ ffmpeg -i some.mp4 -f yuv4mpegpipe | vpxenc [options]

 Useful info in that case if you want to do twopass is to specify:

 ... --passes=2 --pass=1 --fpf=stats.fpf
 ... --passes=2 --pass=2 --fpf=stats.fpf

 on the first and second passes, respectively.

 If using y4m, don't specify the width, height, or framerate (--fps),
 let it parse those out automatically.
"

Original issue reported on code.google.com by jz...@google.com on 30 Sep 2011 at 9:13

[GoogleCodeExporter]

Original comment by louquil...@google.com on 3 Oct 2011 at 8:51

• Changed state: Accepted
• Added labels: ****
• Removed labels: ****

[GoogleCodeExporter]

Original comment by albe...@google.com on 8 Mar 2012 at 12:26

• Changed state: Assigned
• Added labels: ****
• Removed labels: ****