bfin: add README

Based on sh4/README
Suggested-by: jamrial
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/bfin/README

+BFIN optimizations have been removed in
+commit 880e2aa23645ed9871c66ee1cbd00f93c72d2d73
+The last revission with the optimizations is fa4e17c14035ebf43130fb369e1728cdd98d0b72
+
+If you want to maintain these (or other) BFIN optimizations in ffmpeg, then please
+contact ffmpeg-devel@ffmpeg.org