DOC Document requirements for Configurable trait

docs/en/02_Developer_Guides/04_Configuration/00_Configuration.md

@@ -54,12 +54,27 @@ This can be done by calling the static method [Config::inst()](api:SilverStripe\
 $config = Config::inst()->get('MyClass', 'property');
 ```
 
-Or through the `config()` object on the class.
+Or through the `config()` method on the class.
 
 ```php
 $config = $this->config()->get('property')';
 ```
 
+You may need to apply the [Configurable](api:SilverStripe\Core\Config\Configurable) trait in order to access the `config()` method.
+
+**mysite/code/MyOtherClass.php**
+
+```php
+use SilverStripe\Core\Config\Configurable;
+
+class MyOtherClass 
+{
+    use Configurable;
+ 
+}
+```
+
+
 Note that by default `Config::inst()` returns only an immutable version of config. Use `Config::modify()`
 if it's necessary to alter class config. This is generally undesirable in most applications, as modification
 of the config can immediately have performance implications, so this should be used sparingly, or