Add a Github document source

[sylvinus]

They mention opening their data:
https://github.com/blog/2201-making-open-source-data-more-available

I'm not sure if the dumps are publicly accessible outside of BigQuery? If not, is using the API the only solution?

[chaconnewu]

GitHub data is available here: https://www.githubarchive.org/ . I believe it is the source for the GitHub data in BigQuery as well.

[sylvinus]

@chaconnewu it seems that GitHub Archive has the events data but not the repository or file data, which is unfortunately what we're mostly interested in at this point :-(