Improve docs #8
Description
- Explain how to retrieve response values https://github.com/pepperize/cdk-github/blob/main/API.md#@pepperize/cdk-github.GithubCustomResource.getAtt
- Add some more examples like setting up members, teams, ...
- Links to API Reference https://github.com/pepperize/cdk-github/blob/main/API.md from Readme
- Add a better usage section to README