minor #11346 [Frontend] Add section for using JSX with Vue.js (Kocal)

This PR was squashed before being merged into the 3.4 branch (closes #11346).

Discussion
----------

[Frontend] Add section for using JSX with Vue.js

Hi :)

This PR add a new section for using JSX with Vue.js:
```js
Encore.useVueLoader(() => {}, {
    useJsx: true
});
```

We should not merge it before symfony/webpack-encore#553 is merged.

Commits
-------

916ad73 [Frontend] Add section for using JSX with Vue.js

Author: javiereguiluz

frontend/encore/vuejs.rst

@@ -40,7 +40,118 @@ updated styles still requires a page refresh.
 
 See :doc:`/frontend/encore/dev-server` for more details.
 
+JSX Support
+-----------
+
+You can enable `JSX with Vue.js`_ by configuring the 2nd parameter of the ``.enableVueLoader()`` method:
+
+.. code-block:: diff
+
+    // webpack.config.js
+    // ...
+
+    Encore
+        // ...
+        .addEntry('main', './assets/main.js')
+
+    -     .enableVueLoader()
+    +     .enableVueLoader(() => {}, {
+    +         useJsx: true
+    +     })
+    ;
+
+Next, run or restart Encore. When you do, you will see an error message helping you
+install any missing dependencies. After running that command and restarting
+Encore, you're done!
+
+Your ``.jsx`` files will now be transformed through ``@vue/babel-preset-jsx``.
+
+Using styles
+~~~~~~~~~~~~
+
+You can't use ``<style>`` in ``.jsx`` files.
+As a workaround, you can import ``.css``, ``.scss``, etc, files manually:
+
+.. code-block:: javascript
+
+    // App.jsx
+
+    import './App.css'
+
+    export default {
+        name: 'App',
+        render() {
+            return (
+                <div>
+                    ...
+                </div>
+            )
+        }
+    }
+
+.. note::
+
+    Importing styles this way make them global.
+    See the next section for scoping them to your component.
+
+Using Scoped Styles
+~~~~~~~~~~~~~~~~~~~
+
+You also can't use `Scoped Styles`_ (``<style scoped>``) in ``.jsx`` files.
+As a workaround, you can use `CSS Modules`_ by suffixing import paths with ``?module``:
+
+.. code-block:: javascript
+
+    // Component.jsx
+
+    import styles from './Component.css?module' // suffix with "?module"
+
+    export default {
+        name: 'Component',
+        render() {
+            return (
+                <div>
+                    <h1 class={styles.title}>
+                        Hello World
+                    </h1>
+                </div>
+            )
+        }
+    }
+
+.. code-block:: css
+
+    /* Component.css */
+
+    .title {
+        color: red
+    }
+
+The output will be something like ``<h1 class="h1_a3dKp">Hello World</h1>``.
+
+Using images
+~~~~~~~~~~~~
+
+You can't use ``<img src="./image.png">`` in ``.jsx`` files.
+As a workaround, you can import them with ``require()`` function:
+
+.. code-block:: javascript
+
+    export default {
+        name: 'Component',
+        render() {
+            return (
+                <div>
+                    <img src={require("./image.png")} />
+                </div>
+            )
+        }
+    }
+
 .. _`babel-preset-react`: https://babeljs.io/docs/plugins/preset-react/
 .. _`Vue.js`: https://vuejs.org/
 .. _`vue-loader options`: https://vue-loader.vuejs.org/options.html
 .. _`Encore's index.js file`: https://github.com/symfony/webpack-encore/blob/master/index.js
+.. _`JSX with Vue.js`: https://github.com/vuejs/jsx
+.. _`Scoped Styles`: https://vue-loader.vuejs.org/guide/scoped-css.html
+.. _`CSS Modules`: https://github.com/css-modules/css-modules