sane-backends: mention in help message which libraries may be selected

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/sane-backends/Config.in

@@ -4,3 +4,7 @@ config BR2_PACKAGE_SANE_BACKENDS
 	  SANE - Scanner Access Now Easy
 
 	  http://www.sane-project.org
+
+	  Backends are included automatically based on the libraries that have
+	  been selected: libusb, jpeg, tiff, avahi (with dbus and libglib2),
+	  and netsnmp.