add on-prompt docs to README

README

@@ -387,6 +387,40 @@ WHAT ELSE CAN IT DO?
     The other template editor actions are "show", "list", and "delete".
 
 
+  NESTABLE PROMPT COMMANDS
+
+    Users can define their own actions which will be taken just before each
+    prompt is displayed.  In fact, smartcd itself can use this mechanism to
+    invoke itself.
+
+    smartcd on-prompt takes this concept to a new level.  By specifying commands
+    to be run, they will be run on each prompt, similar to the built-in feature,
+    but only while you are in the current directory and its children.  In
+    addition to this, further on-prompt commands specified in child directories
+    will run in addition to the current ones rather than replacing them.
+
+    For example:
+
+        ~/foo$ smartcd on-prompt 'echo test 1'
+        test 1
+        ~/foo$ cd bar
+        test 1
+        ~/foo/bar$ smartcd on-prompt 'echo test 2'
+        test 1
+        test 2
+        ~/foo/bar$ cd ..
+        test 1
+        ~/foo$ cd
+        ~$
+
+    One way that someone might use this is in concert with the history helper,
+    to flush and optionally reload their history on each prompt, to facilitate
+    cross-terminal sharing.
+
+        smartcd helper run history localize __PATH__/.history
+        smartcd on-prompt 'history -a; history -n'
+
+
   EXIT HOOK
 
     By default, your enter scripts will be run when a shell is started.  However,