Add docs to README.rst about pavlova parsers

README.rst

@@ -78,6 +78,57 @@ Usage with Flask
 
     app.run()
 
+Adding Custom Types
+###################
+
+There are a couple of different ways to implement new types for parsing in
+pavlova. In general, the process is to add a parser a specific type. For
+validation you should raise a TypeError or ValueError.
+
+The first one, is creating a new type that extends an existing base type. Here
+is an example on how to implement an Email type, which is a string but performs
+validation.
+
+.. code-block:: python
+
+    from pavlova import Pavlova
+    from pavlova.parsers import GenericParser
+
+    class Email(str):
+        def __new__(cls, input_value: typing.Any) -> str:
+            if isinstance(input_value, str):
+                if '@' in input_value:
+                    return str(input_value)
+                raise ValueError()
+            raise TypeError()
+
+    pavlova = Pavlova()
+    pavlova.register_parser(Email, GenericParser(pavlova, Email))
+
+Another way, is to implement your own pavlova parser, rather than using your
+the built in `GenericParser` parser.
+
+.. code-block:: python
+
+    import datetime
+    from typing import Any, Tuple
+
+    import dateparser
+    from pavlova import Pavlova
+    from pavlova.parsers import PavlovaParser
+
+    class DatetimeParser(PavlovaParser[datetime.datetime]):
+        "Parses a datetime"
+
+        def parse_input(self,
+                        input_value: Any,
+                        field_type: Type,
+                        path: Tuple[str, ...]) -> datetime.datetime:
+            return dateparser.parse(input_value)
+
+    pavlova = Pavlova()
+    pavlova.register_parser(datetime.DateTime, DatetimeParser(pavlova))
+
 Requirements
 ############