wrap-handlers: add docs

README.md

@@ -51,17 +51,26 @@ Handlers can be:
   `action` is fired.
 - __onStateChange(data, state, prev, caller, createSend):__ called after a reducer
   changes the `state`.
+- __wrapSubscriptions(fn):__ wraps a `subscription` to add custom behavior
+- __wrapReducers(fn):__ wraps a `reducer` to add custom behavior
+- __wrapEffects(fn):__ wraps an `effect` to add custom behavior
+- __wrapInitialState(fn):__ wraps the combined initial `state` to add custom
+  behavior - useful to mutate the state before starting up
 
 `createSend()` is a special function that allows the creation of a new named
 `send()` function. The first argument should be a string which is the name, the
 second argument is a boolean `callOnError` which can be set to `true` to call
 the `onError` hook istead of a provided callback. It then returns a
 `send(actionName, data?)` function.
 
-Handlers should be used with care, as they're the most powerful interface into
-the state. For application level code it's generally recommended to delegate to
-actions inside models using the `send()` call, and only shape the actions
-inside the handlers.
+Store-level handlers (`on*`) should be used with care, as they're the most
+powerful interface into the state. For application level code it's generally
+recommended to delegate to actions inside models using the `send()` call, and
+only shape the actions inside the handlers.
+
+Wrappers (`wrap*`) are useful to extend or even completely override how
+`barracks` handlers. For example new asynchronous models (such as `pull-stream`
+or `xstream`) can be applied or the way objects are diffed can be changed.
 
 ### store.model()
 Register a new model on the store. Models are optionally namespaced objects