Improve docs

[pflorek]

• 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