explain new entry behavior

Readme.md

@@ -22,4 +22,13 @@ Write a username and password:
     n["api.heroku.com"] = user, newpass
     n.save
 
+If you make an entry that wasn't there before, it will be appended
+to the end of the file. Sometimes people want to include a comment
+explaining that the entry was added automatically. You can do it
+like this:
+
+    n.new_item_prefix = "# This entry was added automatically\n"
+    n["api.heroku.com"] = user, newpass
+    n.save
+
 Have fun!