Add decimal to cast docs

For laravel/framework#26173

Author: barryvdh

eloquent-mutators.md

@@ -158,7 +158,7 @@ By default, timestamps are formatted as `'Y-m-d H:i:s'`. If you need to customiz
 <a name="attribute-casting"></a>
 ## Attribute Casting
 
-The `$casts` property on your model provides a convenient method of converting attributes to common data types. The `$casts` property should be an array where the key is the name of the attribute being cast and the value is the type you wish to cast the column to. The supported cast types are: `integer`, `real`, `float`, `double`, `string`, `boolean`, `object`, `array`, `collection`, `date`, `datetime`, and `timestamp`.
+The `$casts` property on your model provides a convenient method of converting attributes to common data types. The `$casts` property should be an array where the key is the name of the attribute being cast and the value is the type you wish to cast the column to. The supported cast types are: `integer`, `real`, `float`, `double`, `decimal:<digits>`, `string`, `boolean`, `object`, `array`, `collection`, `date`, `datetime`, and `timestamp`. When casting to `decimal`, you should define the number of digits, eg. `decimal:2`
 
 For example, let's cast the `is_admin` attribute, which is stored in our database as an integer (`0` or `1`) to a boolean value: