Added documentation regarding the newly public timestamp() function

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

laravel/documentation/database/eloquent.md

@@ -137,6 +137,13 @@ In some cases it may be useful to update the **updated_at** date column without
 	$comment = Comment::find(1);
 	$comment->touch();
 
+You can also use the **timestamp** function to update the **updated_at** date column without saving the model immediately. Note that if you are actually modifying the model's data this is handled behind the scenes:
+
+	$comment = Comment::find(1);
+	$comment->timestamp();
+	//do something else here, but not modifying the $comment model data
+	$comment->save();
+
 > **Note:** You can change the default timezone of your application in the **application/config/application.php** file.
 
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