Add docs for no-unused-state

Author: Joachim Seminck

README.md

@@ -108,6 +108,7 @@ Finally, enable all of the rules that you would like to use.  Use [our preset](#
 * [react/no-unescaped-entities](docs/rules/no-unescaped-entities.md): Prevent invalid characters from appearing in markup
 * [react/no-unknown-property](docs/rules/no-unknown-property.md): Prevent usage of unknown DOM property (fixable)
 * [react/no-unused-prop-types](docs/rules/no-unused-prop-types.md): Prevent definitions of unused prop types
+* [react/no-unused-state](docs/rules/no-unused-state.md): Prevent definitions of unused state properties
 * [react/no-will-update-set-state](docs/rules/no-will-update-set-state.md): Prevent usage of `setState` in `componentWillUpdate`
 * [react/prefer-es6-class](docs/rules/prefer-es6-class.md): Enforce ES5 or ES6 class for React Components
 * [react/prefer-stateless-function](docs/rules/prefer-stateless-function.md): Enforce stateless React Components to be written as a pure function

docs/rules/no-unused-state.md

@@ -0,0 +1,45 @@
+# Prevent definitions of unused state (react/no-unused-state)
+
+Warns you if you have defined a property on the state, but it is not being used anywhere.
+
+## Rule Details
+
+The following patterns are considered warnings:
+
+```jsx
+class MyComponent extends React.Component {
+  state = { foo: 0 };
+  render() {
+    return <SomeComponent />;
+  }
+}
+
+var UnusedGetInitialStateTest = createReactClass({
+  getInitialState: function() {
+    return { foo: 0 };
+  },
+  render: function() {
+    return <SomeComponent />;
+  }
+})
+```
+
+The following patterns are not considered warnings:
+
+```jsx
+class MyComponent extends React.Component {
+  state = { foo: 0 };
+  render() {
+    return <SomeComponent foo={this.state.foo} />;
+  }
+}
+
+var UnusedGetInitialStateTest = createReactClass({
+  getInitialState: function() {
+    return { foo: 0 };
+  },
+  render: function() {
+    return <SomeComponent foo={this.state.foo} />;
+  }
+})
+```