Custom list filter documentation.

Author: Aitor Brazaola

docs/custom_list_filter.md

@@ -3,10 +3,31 @@
 A list filter is an implementation that defines how an a filter input has to be rendered. This works together with the
 `AdminRepository` that receives the submitted form that was built using list filter types.
 
+Using filters, you can implement customized search based on any field of your entities, although those already 
+provided `text` and `date` may be enough for most project, perhaps you will need to customize or create more complex 
+filters for your project specific business logic. 
+
 1. Create a new class implementing `LIN3S\AdminBundle\Configuration\Type\ListFilterType`
 1. Add the class to Dependency Container with `lin3s_admin.list_filter_type` tag name and the desired alias.
 1. Reference the created service with the alias in the property `type` of list > filters > field_name of your `admin.yml`
 
 ## Render function
 
->TODO
+You can handle the header representation of the row within this function, these are the parameters available:
+* filter: A `ListFilter` entity of the filter.
+* currentValue: The value to be filtered.
+* options: An array of options.
+
+Text filter example:
+
+```php
+    $attributes = $currentValue !== null ? ' value="' . $currentValue . '"' : '';
+    
+    foreach ($options['attrs'] as $attrName => $attr) {
+        $attributes .= ' ' . $attrName . '="' . $attr . '"';
+    }
+    
+    return sprintf('<input %s type="text" data-filter-field="%s">', $attributes, $filter->field());
+```
+
+