Added custom list field docs.

Author: Aitor Brazaola

docs/custom_list_field.md

@@ -1 +1,29 @@
 # Creating a custom list field
+
+In case you want to write your own list field column rendered you need to implement
+`LIN3S\AdminBundle\ListFields\ListFieldType`. The `render()` method is expected to return an already escaped
+string, can be HTML but be careful as it will be printed in the list without been escaped.
+
+ListFieldTypes are usually generic and need to be associated with an entity in the configuration.
+
+1. Create a new class implementing `LIN3S\AdminBundle\ListFields\ListFieldType`
+1. Add the class to Dependency Container with `lin3s_admin.list_field_type` tag name and the desired alias.
+1. Reference the created service with the alias in the property `type` of list > field > field_name of your `admin.yml`
+
+## Header function
+
+You can handle the header representation of the column within this function, these are the parameters available:
+* options: Array with the name of the field.
+* configuration: The configuration.
+
+## Render function
+
+You can handle the header representation of the row within this function, these are the parameters available:
+* entity: The entity to represent in the row.
+* options: Array with the options declared in admin.yml
+* configuration: The configuration.
+
+## Translation
+
+Translating header or rendered row content is possible adding the argument `"@translator.default"` to receive an 
+instance of  translator ready to use.