Add doc for state-in-constructor rule

Author: lukyth

docs/rules/state-in-constructor.md

@@ -0,0 +1,74 @@
+# Enforce state initialization style (react/state-in-constructor)
+
+This rule will enforce the state initialization style to be either in a constructor or with a class property.
+
+## Rule Options
+
+```js
+...
+"react/state-in-constructor": [<enabled>, <mode>]
+...
+```
+
+### `always` mode
+
+Will enforce the state initialization style to be in a constructor. This is the default mode.
+
+The following patterns are considered warnings:
+
+```jsx
+class Foo extends React.Component {
+  state = { bar: 0 }
+  render() {
+    return <div>Foo</div>
+  }
+}
+```
+
+The following patterns are **not** considered warnings:
+
+```jsx
+class Foo extends React.Component {
+  constructor(props) {
+    super(props)
+    this.state = { bar: 0 }
+  }
+  render() {
+    return <div>Foo</div>
+  }
+}
+```
+
+### `never` mode
+
+Will enforce the state initialization style to be with a class property.
+
+The following patterns are considered warnings:
+
+```jsx
+class Foo extends React.Component {
+  constructor(props) {
+    super(props)
+    this.state = { bar: 0 }
+  }
+  render() {
+    return <div>Foo</div>
+  }
+}
+```
+
+The following patterns are **not** considered warnings:
+
+```jsx
+class Foo extends React.Component {
+  state = { bar: 0 }
+  render() {
+    return <div>Foo</div>
+  }
+}
+```
+
+
+## When Not To Use It
+
+When the way a component state is being initialized doesn't matter.