Add module documentation.

modules/helloworld/README.md

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+# Module: Hello World
+The `helloworld` module is one of the default modules of the MagicMirror. It is a simple way to display a static text on the mirror.
+## Using the module
+
+To use this module, add it to the modules array in the `config/config.js` file:
+````javascript
+modules: [
+	{
+		module: 'helloworld',
+		position: 'bottom_bar',	// This can be any of the regions.
+		config: {
+			// See 'Configuration options' for more information. 
+			text: 'Hello world!',
+		}
+	}
+]
+````
+
+## Configuration options
+
+The following properties can be configured:
+
+
+<table width="100%">
+	<!-- why, markdown... -->
+	<thead>
+		<tr>
+			<th>Option</th>
+			<th width="100%">Description</th>
+		</tr>
+	<thead>
+	<tbody>
+
+		<tr>
+			<td><code>text</code></td>
+			<td>The text to display.<br>
+				<br><b>Example:</b> <code>'Hello world!'</code>
+				<br><b>Default value:</b> <code>'Hello world!'</code>
+			</td>
+		</tr>
+		<tr>
+			<td><code>classes</code></td>
+			<td>Classes to apply to the text.<br>
+				<br><b>Example:</b> <code>'xsmall bold'</code>
+				<br><b>Default value:</b> <code>'normal medium'</code>
+			</td>
+		</tr>
+
+		
+	</tbody>
+</table>