diff --git a/doc/README.developer b/doc/README.developer
index 3cb73d60fd..09dc5838fe 100644
--- a/doc/README.developer
+++ b/doc/README.developer
@@ -3241,6 +3241,14 @@ This will create preferences "beep.tcp.port" and
 "beep.strict_header_terminator", the first of which is an unsigned
 integer and the second of which is a Boolean.
 
+Note that a warning will pop up if you've saved such preference to the 
+preference file and you subsequently take the code out. The way to make 
+a preference obsolete is to register it as such:
+
+/* Register a preference that used to be supported but no longer is. */
+	void prefs_register_obsolete_preference(module_t *module,
+	    const char *name);
+
 2.7 Reassembly/desegmentation for protocols running atop TCP.
 
 There are two main ways of reassembling a Protocol Data Unit (PDU) which