I am using the Flickr4Java library, which is hosted on GitHub. I have a question to the authors. On the main page it is written:
Comments and questions should be sent to the GitHub Repo.
How do I do that?
I guess they meant that the comments and questions should be sent to the issue tracker, in this case https://github.com/callmeal/Flickr4Java/issues.
For issues with the the code you should use https://github.com/callmeal/Flickr4Java/issues.
However, to reach out to the author/maintainer with something not code related or bug, you should find out if he has his email or equivalent public.
For this particular library for example, you can reach out to the author through https://twitter.com/boncey, which is nicer than opening an issue when it's not a code issue.
Choose the repository you'd like to have a question and then in that Repository, create an issue or click Issue for you to have a question on Github.