Added documentation for the touch() function

Signed-off-by: Callum McIntyre <mcintyre1994@gmail.com>

laravel/documentation/database/eloquent.md

@@ -132,6 +132,11 @@ ## Inserting & Updating Models
 
 Next, add **created_at** and **updated_at** date columns to your table. Now, whenever you save the model, the creation and update timestamps will be set automatically. You're welcome.
 
+In some cases it may be useful to update the **updated_at** date column without actually modifying any data within the model. Simply use the **touch** method, which will also automatically save the changes immediately:
+
+	$comment = Comment::find(1);
+	$comment->touch();
+
 > **Note:** You can change the default timezone of your application in the **application/config/application.php** file.
 
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