Update readme file

Author: ahangarha

README.md

@@ -685,14 +685,13 @@ var Post = createReactClass({
 
 The generator also accepts options:
 
-- `--function` or `--func`: use `function ComponentName() {...}`
-- `--es6`: use `class ComponentName extends React.Component`
+- `--es6`: generates a function component
 - `--coffee`: use CoffeeScript
 
 For example,
 
 ```sh
-rails g react:component ButtonComponent title:string --function
+rails g react:component ButtonComponent title:string --es6
 ```
 
 would generate:
@@ -701,7 +700,7 @@ would generate:
 import React from "react"
 import PropTypes from "prop-types"
 
-function ButtonComponent() {
+function ButtonComponent(props) {
   return (
     <React.Fragment>
       Title: {this.props.title}
@@ -714,9 +713,10 @@ ButtonComponent.propTypes = {
 };
 
 export default ButtonComponent
-
 ```
 
+**Note:** In a Webpacker project, es6 template is the default template in the generator.
+
 Accepted PropTypes are:
 
 - Plain types: `any`, `array`, `bool`, `element`, `func`, `number`, `object`, `node`, `shape`, `string`