prefab: Add README.md to explain components separation

prefab/README.md

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+# Prefab
+
+Prefabs are `.bin` or `.glb` files that can be instantiated into the scene:
+
+```js
+const prefab = await engine.loadPrefab('Prefab.bin');
+for (let i = 0; i < 10; ++i) {
+    engine.scene.instantiate(prefab);
+}
+```
+
+For more information about prefabs, have a look at the [Prefab](https://wonderlandengine.com/jsapi/prefab/) documentation.
+
+## Prefab Creation
+
+* Create a new project called `Prefab`
+* Reach the `Views` tab
+    * Click `Project Settings`
+* In the `Project Settings` window
+    * Reach the `Project` section
+    * Tick the `packageForStreaming` box
+
+You can add models, textures, materials, and js components like a normal scene.
+
+## JavaScript Components
+
+The Wonderland Editor doesn't yet have support for automatic prefab management. Thus, you will need to
+help the editor know about how to register your JavaScript/TypeScript components.
+
+Create a file called `prefab-index.js` and add the following template:
+
+_prefab-index.js_:
+
+```js
+/* wle:auto-imports:start */
+/* wle:auto-imports:end */
+
+export default function(engine) {
+    /* wle:auto-register:start */
+    /* wle:auto-register:end */
+}
+```
+
+This template is used by the editor to insert import and registration statements.
+
+Next, edit the entry point in your `Prefab.wlp` file:
+
+* Reach the `Views` tab
+    * Click `Project Settings`
+* In the `Project Settings` window
+    * Reach the `JavaScript` section
+    * Edit the `entryPoint` field to: "prefab-index.js"
+
+Finally, edit the main `index.js` of your application with:
+
+_index.js_:
+
+```js
+import registerPrefabComponents from './prefab-index.js';
+
+...
+
+registerPrefabComponents(engine);
+```
+
+This will allow you to automatically get the prefab js components registered in the application.