update state management article (#443)

docs/get-started/state-management.md

@@ -11,6 +11,41 @@ The most simple handling of this intent is to save it and pass it back to the co
 
 User interaction and transition will not work without a valid `onViewportChange` handler.
 
+The advantage of this practice is that it ensures a single source of truth regarding the viewport state (in the example above, saved in the `state` of the container component). When you use this viewport state to direct the rendering of other components, it is guaranteed that they will always be synced with the map.
+
+You may apply additional constraints to the viewport:
+
+```jsx
+_onViewportChange = viewport => {
+    if (viewport.longitude > 0) {
+        viewport.longitude = 0;
+    }
+    this.setState({viewport});
+}
+
+render() {
+    return <ReactMapGL {...this.state.viewport} onViewportChange={this._onViewportChange} />
+}
+```
+
+Or manipulate the viewport outside of the ReactMap component:
+
+```jsx
+_goToNYC = () => {
+    const viewport = {...this.state, longitude: -74.1, latitude: 40.7};
+    this.setState({viewport});
+}
+
+render() {
+    return (
+        <div>
+            <ReactMapGL {...this.state.viewport} onViewportChange={this._onViewportChange} />
+            <button onClick={this._goToNYC}>New York City</button>
+        </div>
+    );
+}
+```
+
 
 ## Using with Redux