Added docs for map_field()

Closes encode#7252, encode#7140

docs/api-guide/schemas.md

@@ -240,6 +240,29 @@ Not all `AutoSchema` methods expose related  `__init__()` kwargs, but those for
 
 ### `AutoSchema` methods
 
+
+
+#### `map_field()`
+
+Maps individual serializer fields to their schema representation. The base implementation
+will handle the default fields that Django REST Framework provides.
+
+For `SerializerMethodField` instances, for which the schema is unknown, or custom field subclasses you should override `map_field()` to generate the correct schema:
+
+```python
+class CustomSchema(AutoSchema):
+    """Extension of ``AutoSchema`` to add support for custom field schemas."""
+
+    def map_field(self, field):
+        # Handle SerializerMethodFields or custom fields here...
+        # ...
+        return super()map_field(field)
+```
+
+Authors of third-party packages should aim to provide an `AutoSchema` subclass,
+and a mixin overriding `map_field()` so that users can easily generate schemas
+for their custom fields.
+
 ### `AutoSchema.__init__()` kwargs