Update documentation on alert behaviours (#149)

Added the error alert for the alert icons

README.md

@@ -89,6 +89,10 @@ Displaying different alert icons.
 $this->alert('success', 'Success is approaching!');
 ```
 
+``` php
+$this->alert('error', 'Wrong email or password!');
+```
+
 ``` php
 $this->alert('warning', 'The world has warned you.');
 ```