Added a description of the dll.d directory usage to sane-dll manpage

doc/sane-dll.man

@@ -52,6 +52,18 @@ pnm
 .br
 mustek
 .RE
+.PP
+It is also possible to add a file in
+.I @CONFIGDIR@/dll.d
+that contains the list of backends to be added. Backends mentioned in a 
+file included in this directory will be added before any backends listed 
+in 
+.I dll.conf.
+Files in 
+.I @CONFIGDIR@/dll.d
+can be freely named. They shall follow the format conventions as apply for
+.I dll.conf.
+
 .PP
 Note that backends that were pre-loaded when building this library do
 not have to be listed in this configuration file.  That is, if a