Add higher-order store docs

Author: acdlite

docs/higher-order-stores.md

@@ -0,0 +1,57 @@
+Higher-order stores
+===================
+
+A higher-order store is a function that turns a store creating function into a new store creating function:
+
+```
+createStore => createStore'
+```
+
+Look familiar? It's just like the signature for [middleware](middleware.md), only we're wrapping `createStore()` instead of `dispatch()`.
+
+Higher-order stores are a pattern for creating composable Redux extensions. They're much the same as higher-order components in React. They can be as simple as `applyMiddleware()`, or as powerful as the Redux Devtools](https://github.com/gaearon/redux-devtools). There's no limit to the kinds of things you can create.
+
+## How it works
+
+Let's look at an example. As alluded to above, `applyMiddleware()` is an example of a higher-order store. You use it by wrapping the base `createStore()` function provided by Redux:
+
+```js
+const newCreateStore = applyMiddleware(m1, m2, m3)(createStore);
+```
+
+Internally, `applyMiddleware()` works by proxying the `dispatch()` method returned by `createStore()`:
+
+```js
+// Implementation has been simplified for the purpose of illustration
+export default function applyMiddleware(...middlewares) {
+  // ...combine middlewares...
+
+  return next => (reducer, initialState) => {
+    const store = next(reducer, initialState);
+    return {
+      ...store,
+      dispatch: middleware(store.dispatch)
+    };
+  };
+}
+```
+
+`next` is the next store creating function in the chain — either the return value of another higher-order store, or `createStore()` itself.
+
+This design allows multiple higher-stores can be combined simply using function composition:
+
+```js
+const newCreateStore = compose(
+  applyMiddleware(m1, m2, m3),
+  devTools,
+  createStore
+);
+```
+
+Now just pass your reducer (and an initial state, if desired) to your new store creating function:
+
+```js
+const store = createStore(reducer, intialState);
+
+<Provider store={store} />
+```