[mythtv] transcoder documentation.

[Chris Petersen]

> A note of caution: I wouldn't delete your old files immediately.  The
> method I chose for adjusting key-frames due to the commercial cut works
> great for me, but it will generate files which are incompatible with
> mencoder (until the mencoder patch is updated).

This does, however, still work with mplayer.  which is all I need to
archive to svcd.  awesome.  Now if only I could get the transcoder
working from within mythfrontend.  X doesn't seem to be working for me. 
maybe I need to enable something in the db?

-Chris