Add documentation for custom formatters

Author: chreke

README.rst

@@ -177,6 +177,34 @@ Pass custom defined media type deserializers dictionary with supported mimetypes
 
    result = validator.validate(request, response)
 
+Formats
+*******
+
+OpenAPI defines a `format` keyword that hints at how a value should be interpreted, e.g. a `string` with the type `date` should conform to the RFC 3339 date format.
+
+Openapi-core comes with a set of built-in formatters, but it's also possible to add support for custom formatters for `RequestValidator` and `ResponseValidator`.
+
+Here's how could add support for a `usdate` format that handles dates of the form MM/DD/YYYY:
+
+.. code-block:: python
+    from datetime import datetime
+    import re
+
+    class USDateFormatter:
+        def validate(self, value) -> bool:
+            return bool(re.match(r"^\d{1,2}/\d{1,2}/\d{4}$", value))
+
+        def unmarshal(self, value):
+            return datetime.strptime(value, "%m/%d/%y").date
+
+
+   custom_formatters = {
+       'usdate': USDateFormatter(),
+   }
+
+   validator = ResponseValidator(spec, custom_formatters=custom_formatters)
+
+   result = validator.validate(request, response)
 
 Integrations
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