fill out readme with usage details

Author: chrisbreiding

README.md

@@ -2,14 +2,94 @@
 
 Cypress preprocessor for bundling JavaScript via webpack
 
-## Install
+## Installation
 
 Requires [Node](https://nodejs.org/en/) version 6.5.0 or above.
 
 ```sh
 npm install --save-dev cypress-webpack-preprocessor
 ```
 
+## Usage
+
+In your project's [plugins file](https://on.cypress.io/guides/plugins):
+
+```javascript
+const webpack = require('@cypress/webpack-preprocessor')
+
+module.exports = (register, config) => {
+  register('on:spec:file:preprocessor', webpack(config))
+}
+```
+
+## Options
+
+Pass in options as the second argument to `webpack`:
+
+```javascript
+module.exports = (register, config) => {
+  const options = {
+    // send in the options from your webpack.config.js, so it works the same
+    // as your app's code
+    webpackOptions: require('../../webpack.config'),
+    watchOptions: {},
+  }
+
+  register('on:spec:file:preprocessor', webpack(config, options))
+}
+```
+
+### webpackOptions
+
+Object of webpack options. Just `require` in the options from your `webpack.config.js` to use the same options as your app.
+
+**Default**:
+
+```javascript
+{
+  module: {
+    rules: [
+      {
+        test: /\.jsx?$/,
+        exclude: [/node_modules/],
+        use: [{
+          loader: 'babel-loader',
+          options: {
+            presets: [
+              'babel-preset-env',
+              'babel-preset-react',
+            ],
+          },
+        }],
+      },
+    ],
+  },
+}
+```
+
+### watchOptions
+
+Object of options for watching. See [webpack's docs](https://webpack.github.io/docs/node.js-api.html#compiler).
+
+**Default**: `{}`
+
+## Modifying default options
+
+The default options are provided as `webpack.defaultOptions` so they can be more easily modified.
+
+If, for example, you want to update the options for the `babel-loader` to add the [stage-3 preset](https://babeljs.io/docs/plugins/preset-stage-3/), you could do the following:
+
+```javascript
+const webpack = require('@cypress/webpack-preprocessor')
+
+module.exports = (register, config) => {
+  const options = webpack.defaultOptions
+  options.webpackOptions.module.rules[0].use.options.presets.push('babel-preset-stage-3')
+
+  register('on:spec:file:preprocessor', webpack(config, options))
+}
+```
+
 ## License
 
 This project is licensed under the terms of the [MIT license](/LICENSE.md).