Add contribution guide and PR template for contributors

[ddkluchnikov]

Hello 👋

Thank you for the great SDK — it looks very useful. I noticed there is no CONTRIBUTING.md or PR template to guide new contributors.

Would you be open to adding:

• a CONTRIBUTING.md that explains how to add examples and SDK changes,
• a PR template for small docs/examples fixes,
• a simple TEMPLATE.md for new example entries (if applicable)?

If yes, I can prepare a PR that adds:

• CONTRIBUTING.md (basic contribution steps),
• .github/PULL_REQUEST_TEMPLATE.md,
• docs/TEMPLATE.md for example contributions.

This should make it easier for new contributors to submit useful PRs.

Thanks! :)