debian: Update README file in doc package

Signed-off-by: Philippe Coval <rzr@users.sf.net>
Change-Id: I7cd02e3094cb60328a24702afecb13f61beb67a1

debian/docs

@@ -1,4 +1,4 @@
 BUGS.txt
 CREDITS.txt
-README.txt
+README.md
 README-SDL.txt