Mention the new per-instance memoization API

docs/api.md

@@ -58,10 +58,14 @@ Instead, it *returns* a new, connected component class, for you to use.
 
 #### Arguments
 
-* [`mapStateToProps(state, [ownProps]): stateProps`] \(*Function*): If specified, the component will subscribe to Redux store updates. Any time it updates, `mapStateToProps` will be called. Its result must be a plain object, and it will be merged into the component’s props. If you omit it, the component will not be subscribed to the Redux store. If `ownProps` is specified as a second argument, its value will be the props passed to your component, and `mapStateToProps` will be re-invoked whenever the component receives new props.
+* [`mapStateToProps(state, [ownProps]): stateProps`] \(*Function*): If specified, the component will subscribe to Redux store updates. Any time it updates, `mapStateToProps` will be called. Its result must be a plain object*, and it will be merged into the component’s props. If you omit it, the component will not be subscribed to the Redux store. If `ownProps` is specified as a second argument, its value will be the props passed to your component, and `mapStateToProps` will be re-invoked whenever the component receives new props.
+
+  >Note: in advanced scenarios where you need more control over the rendering performance, `mapStateToProps()` can also return a function. In this case, *that* function will be used as `mapStateToProps()` for a particular component instance. This allows you to do per-instance memoization. You can refer to [#279](https://github.com/rackt/react-redux/pull/279) and the tests it adds for more details. Most apps never need this.
 
 * [`mapDispatchToProps(dispatch, [ownProps]): dispatchProps`] \(*Object* or *Function*): If an object is passed, each function inside it will be assumed to be a Redux action creator. An object with the same function names, but bound to a Redux store, will be merged into the component’s props. If a function is passed, it will be given `dispatch`. It’s up to you to return an object that somehow uses `dispatch` to bind action creators in your own way. (Tip: you may use the [`bindActionCreators()`](http://rackt.github.io/redux/docs/api/bindActionCreators.html) helper from Redux.) If you omit it, the default implementation just injects `dispatch` into your component’s props. If `ownProps` is specified as a second argument, its value will be the props passed to your component, and `mapDispatchToProps` will be re-invoked whenever the component receives new props.
 
+  >Note: in advanced scenarios where you need more control over the rendering performance, `mapDispatchToProps()` can also return a function. In this case, *that* function will be used as `mapDispatchToProps()` for a particular component instance. This allows you to do per-instance memoization. You can refer to [#279](https://github.com/rackt/react-redux/pull/279) and the tests it adds for more details. Most apps never need this.
+
 * [`mergeProps(stateProps, dispatchProps, ownProps): props`] \(*Function*): If specified, it is passed the result of `mapStateToProps()`, `mapDispatchToProps()`, and the parent `props`. The plain object you return from it will be passed as props to the wrapped component. You may specify this function to select a slice of the state based on props, or to bind action creators to a particular variable from props. If you omit it, `Object.assign({}, ownProps, stateProps, dispatchProps)` is used by default.
 
 * [`options`] *(Object)* If specified, further customizes the behavior of the connector.