README: add markdown syntax

Reviewed-by: llogan
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

README → README.md

 FFmpeg README
--------------
+=============
 
 1) Documentation
 ----------------
 
 * Read the documentation in the doc/ directory in git.
+
   You can also view it online at http://ffmpeg.org/documentation.html
 
 2) Licensing