Started with custom list filter docs

Author: Aitor Brazaola

docs/custom_list_filter.md

@@ -1 +1,12 @@
 # Creating a custom list filter
+
+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.
+
+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