add customization doc (#80)

docs/guide-customization.md

@@ -0,0 +1,31 @@
+---
+id: guide-customization
+title: Customization
+---
+
+CRA Universal features zero config by default, but you can still override the default config with configuration file.
+
+1. Create `crau.config.js` file in the root of your CRA
+2. Here is the starting template
+   ```js
+   module.exports = {
+     modifyWebpack: config => config
+   }
+   ```
+
+## Configurable options
+### modifyWebpack
+You can override default webpack configuration with this callback.
+
+Example:
+```js
+modifyWebpack: config => {
+  const newConfig = {
+    ...config,
+    entry: './server/app.js'
+  };
+  return newConfig;
+}
+```
+
+> It doesn't assume any merging strategy, so it uses the returned output **directly**

website/sidebars.json

@@ -1,7 +1,7 @@
 {
   "docs": {
     "Introduction": ["intro-getting-started"],
-    "Guides": ["guide-deployment"],
+    "Guides": ["guide-deployment", "guide-customization"],
     "API": []
   },
   "docs-other": {