Update readme to include CSS modules support

Author: Vpr99

README.md

@@ -81,6 +81,42 @@ This plugin, unlike the original PurifyCSS plugin, provides special features, su
 
 > The plugin does **not** emit sourcemaps even if you enable `sourceMap` option on loaders!
 
+<h2 align="center">Usage with CSS Modules</h2>
+
+PurifyCSS doesn't support classes that have been namespaced with CSS Modules. However, by adding a static string to `css-loader`'s `localIdentName`, you can effectively whitelist these namespaced classes.
+
+In this example, `PURIFY` will be our whitelisted string. **Note:** Make sure this string doesn't occur in any of your other CSS class names. Keep in mind that whatever you choose will end up in your application at runtime - try to keep it short!
+
+```javascript
+module.exports = {
+  module: {
+    rules: [
+      {
+        test: /\.css$/,
+        loader: ExtractTextPlugin.extract({
+          fallback: 'style-loader',
+          use: [{
+            loader: 'css-loader',
+            query: {
+              localIdentName: 'PURIFY_[hash:base64:5]',
+              modules: true
+            }
+          }]
+        })
+      }
+    ]
+  },
+  plugins: [
+    ...,
+    new PurifyCSSPlugin({
+      purifyOptions: {
+        whitelist: ['*PURIFY*']
+      }
+    })
+  ]
+};
+```
+
 <h2 align="center">Maintainers</h2>
 
 <table>