Remove deprecated require-extension rule from README

Author: yannickcr

README.md

@@ -100,7 +100,6 @@ Finally, enable all of the rules that you would like to use.  Use [our preset](#
 * [react/prefer-stateless-function](docs/rules/prefer-stateless-function.md): Enforce stateless React Components to be written as a pure function
 * [react/prop-types](docs/rules/prop-types.md): Prevent missing props validation in a React component definition
 * [react/react-in-jsx-scope](docs/rules/react-in-jsx-scope.md): Prevent missing `React` when using JSX
-* [react/require-extension](docs/rules/require-extension.md): Restrict file extensions that may be required
 * [react/require-optimization](docs/rules/require-optimization.md): Enforce React components to have a shouldComponentUpdate method
 * [react/require-render-return](docs/rules/require-render-return.md): Enforce ES5 or ES6 class for returning value in render function
 * [react/self-closing-comp](docs/rules/self-closing-comp.md): Prevent extra closing tags for components without children