how to get in the training set (#34424)

* how to get in the training set

* Update CONTRIBUTING.md

* Update CONTRIBUTING.md

* Update docs/CONTRIBUTING.md

docs/CONTRIBUTING.md

@@ -63,3 +63,12 @@ A good pull request has all of the following:
 * Connect your device to Wi-Fi regularly, so that we can pull data for training better driving models.
 * Run the `nightly` branch and report issues. This branch is like `master` but it's built just like a release.
 * Annotate images in the [comma10k dataset](https://github.com/commaai/comma10k).
+
+## Contributing Training Data
+
+### A guide for forks
+
+In order for your fork's data to be eligible for the training set:
+* **Your cereal messaging structs must be [compatible](../cereal#custom-forks)**
+* **The definitions of all the stock messaging structs must not change**: Do not change how any of the fields are set, including everything from `selfdriveState.enabled` to `carState.steeringAngleDeg`. Instead, create your own structs and set them however you'd like.
+* **Do not include cars that are not supported in upstream platforms**: Instead, create new opendbc platforms for cars that you'd like to support outside of upstream, even if it's just a trim-level difference.