Read more details at https://www.neos.io/news/neos-is-now-on-github.html and find all repositories at https://github.com/neos/. Now, before you go there, clone everything and start creating pull requests, maybe go and read https://www.neos.io/develop/contribute.html
Thanks to everyone (@daniellienert, @christianm, @sebastian, …) involved with the move!