Update docs regarding writing optimizations:
- mention clobber-marking of xmm registers,
- some notes on external vs. inline asm, including tips on which to use for
what situation and to not rewrite+improve in the same patch (as with C code)
- some more best-practice guidelines
See "[PATCH] update doc/optimization.txt" thread on ML.
Originally committed as revision 25170 to svn://svn.ffmpeg.org/ffmpeg/trunk