I've posted the release process (e.g., branching plans) for Swift 5.0 on the Swift.org blog:
https://swift.org/blog/5-0-release-process/
A key date to keep in mind is that November 16, 2018 is the target date for the final re-branch of swift-5.0-branch (the release branch of Swift 5) from master. Changes can be cherry-picked in after that, but essentially the release will be in steady convergence from that point forward.