Adding an intro to chapter 4

Author: tjvantoll

index.html

@@ -1030,7 +1030,18 @@ <h4 class="exercise-start">
 			</div>
 			<div class="chapter">
 				<h2 id="nativescript-modules">NativeScript modules</h2>
-<p>Explain what modules are. Should be able to copy content almost verbatim from the current guide.</p>
+<p>In this chapter you&#39;ll learn about NativeScript modules, which are the JavaScript modules in your app&#39;s <code>node_modules/tns-core-modules</code> folder. Whether you&#39;ve realized it or not, you&#39;ve already used several NativeScript modules, as all of the NativeScript UI elements are actually implemented with JavaScript code.</p>
+<p>If you dig into <code>node_modules/tns-core-modules</code> you can get an idea of how these modules work. Start by finding the <code>node_modules/tns-core-modules/camera</code> folder, which includes the implementation of the camera module. It includes:</p>
+<ul>
+<li>a <code>package.json</code> file that sets the name of the module;</li>
+<li>a file containing the module&#39;s Android implementation (<code>camera.android.js</code>);</li>
+<li>a file containing the module&#39;s iOS implementation (<code>camera.ios.js</code>);</li>
+<li>a file containing code shared by the Android and iOS implementations (<code>camera-common.js</code>)</li>
+</ul>
+<blockquote>
+<p><strong>NOTE</strong>: You can refer to the <a href="https://nodejs.org/api/modules.html#modules_folders_as_modules">Node.js documentation on folders as modules</a> for more detailed information on how NativeScript organizes its modules.</p>
+</blockquote>
+<p>The *.ios.* and *.android.* naming convention should look familiar, as it’s the exact same convention we used to include Android- and iOS-specific styling in <a href="#css">chapter 2.3</a>. NativeScript uses this same convention to implement its modules on iOS and Android. Now that you know where these modules are, let&#39;s take a closer look at what else they can do for your app, starting with a closer looks at what you can do with NativeScript’s UI elements.</p>
 <h3 id="ui-elements">UI elements</h3>
 <p>Explain that UI elements are actually NativeScript modules. Say that we’re going to make this app look nice—it’s a native app after all.</p>
 <h4 class="exercise-start">

src/chapters/chapter4.md

@@ -1,6 +1,17 @@
 ## NativeScript modules
 
-Explain what modules are. Should be able to copy content almost verbatim from the current guide.
+In this chapter you'll learn about NativeScript modules, which are the JavaScript modules in your app's `node_modules/tns-core-modules` folder. Whether you've realized it or not, you've already used several NativeScript modules, as all of the NativeScript UI elements are actually implemented with JavaScript code.
+
+If you dig into `node_modules/tns-core-modules` you can get an idea of how these modules work. Start by finding the `node_modules/tns-core-modules/camera` folder, which includes the implementation of the camera module. It includes:
+
+- a `package.json` file that sets the name of the module;
+- a file containing the module's Android implementation (`camera.android.js`);
+- a file containing the module's iOS implementation (`camera.ios.js`);
+- a file containing code shared by the Android and iOS implementations (`camera-common.js`)
+
+> **NOTE**: You can refer to the [Node.js documentation on folders as modules](https://nodejs.org/api/modules.html#modules_folders_as_modules) for more detailed information on how NativeScript organizes its modules.
+
+The \*.ios.\* and \*.android.\* naming convention should look familiar, as it’s the exact same convention we used to include Android- and iOS-specific styling in [chapter 2.3](#css). NativeScript uses this same convention to implement its modules on iOS and Android. Now that you know where these modules are, let's take a closer look at what else they can do for your app, starting with a closer looks at what you can do with NativeScript’s UI elements.
 
 ### UI elements