added CREDITS file (tell me if I forgot someone)

Originally committed as revision 1235 to svn://svn.ffmpeg.org/ffmpeg/trunk

CREDITS

+This file contains the name of the people who have contributed to
+ffmpeg. The names are sorted alphabetically.
+
+Fabrice Bellard
+Arpad Gereoffy
+Philip Gladstone
+Falk Hueffner
+Zdenek Kabelac
+Nick Kurshev
+Michael Niedermayer
+Franois Revol
+Dieter Shirley
+Juan J. Sierralta
+Lionel Ulmer