[AssetMapper] Replace ImportMapConfigReader by interface

Author: dfernandes-wanadev

frontend/asset_mapper.rst

@@ -955,6 +955,21 @@ rendered by the ``{{ importmap() }}`` Twig function:
             importmap_script_attributes:
                 crossorigin: 'anonymous'
 
+``framework.asset_mapper.config_reader_id``
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Sometimes you may want to create your own config reader to load your importmap or use a custom one from a 3rd party package. You can do this by creating a service that implements ``ImportMapConfigReaderInterface`` and then setting the ``config_reader_id`` option:
+
+.. code-block:: yaml
+
+    framework:
+        asset_mapper:
+            config_reader_id: App\MyCustomImportMapConfigReader
+
+.. tip::
+
+    Don't forget to define your service in your ``services.yaml`` file if further configuration is needed.
+
 Page-Specific CSS & JavaScript
 ------------------------------