Add documentation.

modules/clock/README.md

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+# Module: Clock
+The `clock` module is one of the default modules of the MagicMirror.
+This module displays the current date and time. The information will be updated realtime.
+
+## Using the module
+
+To use this module, add it to the modules array in the `config/config.js` file:
+````javascript
+modules: [
+	{
+		module: 'clock',
+		position: 'top_left',	// This can be any of the regions.
+		config: {
+			// The config property is optional.
+			// See 'Configuration options' for more information. 
+		}
+	}
+]
+````
+
+## Configuration options
+
+The following properties can be configured:
+
+<table>
+	<!-- why, markdown... -->
+	<thead>
+		<tr>
+			<th>Option</th>
+			<th>Description</th>
+		</tr>
+	<thead>
+	<tbody>
+		<tr>
+			<td><code>timeFormat</code></td>
+			<td>Use 12 of 24 hour format.<br>
+				<br><b>Possible values:</b>
+				<br><b>Default value:</b>
+			</td>
+		</tr>
+		<tr>
+			<td><code>displaySeconds</code></td>
+			<td>Display seconds.<br>
+				<br><b>Possible values: <code>true</code> or <code>false</code></b>
+				<br><b>Default value: <code>true</code></b>
+			</td>
+		</tr>
+	</tbody>
+</table>