README: $terminfo does not work under Ubuntu 12.04

This gives further advice for keybindings: for some reason, on my Ubuntu
12.04 system, I need to enter `^[[A` but `$terminfo[kcuu1]` gives `^[OA`

README.md

@@ -42,6 +42,12 @@ Usage
         bindkey -M vicmd 'k' history-substring-search-up
         bindkey -M vicmd 'j' history-substring-search-down
 
+    Ubuntu 12.04 users might need to use this snippet instead:
+
+        # bind UP and DOWN arrow keys
+        bindkey '^[[A' history-substring-search-up
+        bindkey '^[[B' history-substring-search-down
+
 3.  Type any part of any previous command and then:
 
     * Press the UP arrow key to select the nearest command that (1) contains