Add docs for prefer-read-only-props

Author: Luke Zapart

docs/rules/prefer-read-only-props.md

@@ -0,0 +1,47 @@
+# Enforce that props are read-only (react/prefer-read-only-props)
+
+Using Flow, one can define types for props. This rule enforces that prop types are read-only (covariant).
+
+## Rule Details
+
+The following patterns are considered warnings:
+
+```jsx
+type Props = {
+  name: string,
+}
+class Hello extends React.Component<Props> {
+  render () {
+    return <div>Hello {this.props.name}</div>;
+  }
+}
+
+function Hello(props: {-name: string}) {
+  return <div>Hello {props.name}</div>;
+}
+
+const Hello = (props: {|name: string|}) => (
+  <div>Hello {props.name}</div>
+);
+```
+
+The following patterns are **not** considered warnings:
+
+```jsx
+type Props = {
+  +name: string,
+}
+class Hello extends React.Component<Props> {
+  render () {
+    return <div>Hello {this.props.name}</div>;
+  }
+}
+
+function Hello(props: {+name: string}) {
+  return <div>Hello {props.name}</div>;
+}
+
+const Hello = (props: {|+name: string|}) => (
+  <div>Hello {props.name}</div>
+);
+```