added State mixin docs

doc/04 Mixins/State.md

@@ -0,0 +1,37 @@
+Mixes in the `isActive` method of `Router`.
+
+Methods
+-------
+
+### isActive
+
+See [router.isActive][router.isActive]
+
+Example
+-------
+
+Let's say you are using bootstrap and want to get `active` on those `li`
+tags for the Tabs:
+
+```js
+import { Link, State } from 'react-router';
+
+var Tab = React.createClass({
+
+  mixins: [ State ],
+
+  render() {
+    var isActive = this.isActive(this.props.href, this.props.query);
+    var className = isActive ? 'active' : '';
+    return <li className={className}><Link {...this.props}/></li>;
+  }
+
+});
+
+// use it just like <Link/>, and you'll get an anchor wrapped in an `li`
+// with an automatic `active` class on both.
+<Tab href="foo">Foo</Tab>
+```
+
+  [router.isActive]:#TODO
+