docs: add dedicated contributing guides

[marandaneto]

💡 Motivation and Context

This repo had contributor-facing development instructions split between the root README and the SDK compliance adapter README.

This PR moves that guidance into dedicated CONTRIBUTING.md files so contributors have a consistent place to find local setup, integration test, and adapter test instructions.

💚 How did you test it?

• Not run (docs-only changes)
• Checked the updated markdown links and file locations

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran sampo add to generate a changeset file
• Added the release label to the PR

[github-actions]

posthog-elixir Compliance Report

Date: 2026-04-17 10:02:08 UTC
Duration: 222ms

⚠️ Some Tests Failed

0/1 tests passed, 1 failed

---

Feature_Flags Tests

⚠️ 0/1 tests passed, 1 failed

View Details

Test	Status	Duration
Request Payload.Request With Person Properties Device Id	❌	7ms

Failures

request_payload.request_with_person_properties_device_id

404, message='Not Found', url='http://sdk-adapter:8080/get_feature_flag'