Add support for pydantic dataclasses (#406)

README.md

@@ -14,6 +14,7 @@ This project aims to provide an improved experience when using Protobuf / gRPC i
   - Timezone-aware `datetime` and `timedelta` objects
   - Relative imports
   - Mypy type checking
+- [Pydantic Models](https://docs.pydantic.dev/) generation (see #generating-pydantic-models)
 
 This project is heavily inspired by, and borrows functionality from:
 
@@ -364,6 +365,25 @@ datetime.datetime(2019, 1, 1, 11, 59, 58, 800000, tzinfo=datetime.timezone.utc)
 {'ts': '2019-01-01T12:00:00Z', 'duration': '1.200s'}
 ```
 
+## Generating Pydantic Models
+
+You can use python-betterproto to generate pydantic based models, using
+pydantic dataclasses. This means the results of the protobuf unmarshalling will
+be typed checked. The usage is the same, but you need to add a custom option
+when calling the protobuf compiler:
+
+
+```
+protoc -I . --custom_opt=pydantic_dataclasses --python_betterproto_out=lib example.proto
+```
+
+With the important change being `--custom_opt=pydantic_dataclasses`. This will
+swap the dataclass implementation from the builtin python dataclass to the
+pydantic dataclass. You must have pydantic as a dependency in your project for
+this to work.
+
+
+
 ## Development
 
 - _Join us on [Slack](https://join.slack.com/t/betterproto/shared_invite/zt-f0n0uolx-iN8gBNrkPxtKHTLpG3o1OQ)!_