[Encore] Clarification notes about babel

Author: weaverryan

frontend/encore/babel.rst

@@ -19,13 +19,39 @@ Need to extend the Babel configuration further? The easiest way is via
         // first, install any presets you want to use (e.g. yarn add babel-preset-es2017)
         // then, modify the default Babel configuration
         .configureBabel(function(babelConfig) {
+            // add additional presets
             babelConfig.presets.push('es2017');
+
+            // no plugins are added by default, but you can add some
+            // babelConfig.plugins = ['styled-jsx/babel'];
         })
     ;
 
-You can also create a standard ``.babelrc`` file at the root of your project.
-Just make sure to configure it with all the presets you need: as soon as a
-``.babelrc`` is present, Encore can no longer add *any* Babel configuration for
-you!
+Creating a .babelrc File
+------------------------
+
+Instead of calling ``configureBabel()``, you could create a ``.babelrc`` file
+at the root of your project. This is a more "standard" way of configuring
+Babel, but it has a downside: as soon as a ``.babelrc`` file is present,
+**Encore can no longer add any Babel configuration for you**. For example,
+if you call ``Encore.enableReactPreset()``, the ``react`` preset will *not*
+automatically be added to Babel: you must add it yourself in ``.babelrc``.
+
+An example ``.babelrc`` file might look like this:
+
+.. code-block:: json
+
+    {
+        presets: [
+            ['env', {
+                modules: false,
+                targets: {
+                    browsers: '> 1%',
+                    uglify: true
+                },
+                useBuiltIns: true
+            }]
+        ]
+    }
 
 .. _`Babel`: http://babeljs.io/