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 build, sample, and 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

• Added the release label to the PR
• Added exactly one version bump label: bump-patch, bump-minor, or bump-major

[github-actions]

posthog-dotnet Compliance Report

Date: 2026-04-17 10:09:30 UTC
Duration: 266ms

⚠️ 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	❌	42ms

Failures

request_payload.request_with_person_properties_device_id

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

[greptile-apps]

Prompt To Fix All With AI

This is a comment left during a code review.
Path: CONTRIBUTING.md
Line: 50-52

Comment:
**Dropped link to `RELEASING.md`**

The old `README.md` included `For release instructions, see [RELEASING.md](RELEASING.md).` at the end of the Testing section. That line was removed but not carried into `CONTRIBUTING.md`, so release instructions are no longer reachable from any of the changed contributor-facing files. The `## Pull requests` section seems like the natural place to restore it.

```suggestion
Please follow existing conventions and include tests for your change when practical.

For release instructions, see [RELEASING.md](RELEASING.md).
```

How can I resolve this? If you propose a fix, please make it concise.

_{Reviews (1): Last reviewed commit: "docs: add contributing guides" | Re-trigger Greptile}