Add note about debhelper for compiling the debian package. Fixes #520

doc/compiling.md

@@ -82,7 +82,8 @@ Options (do one of these before you plug it in)
 *    2. `modprobe -r usb-storage && modprobe usb-storage`
 
 ### Build Debian Package
-To build debian package you need debuild.
+
+To build the debian package you need the following extra packages: `debuild debhelper`.
 
 ```
 $ git archive --prefix=$(git describe)/ HEAD | bzip2 --stdout > ../libstlink_$(sed -En -e "s/.*\((.*)\).*/\1/" -e "1,1 p" debian/changelog).orig.tar.bz2