Add 'circleci step halt' documentation
I've been looking for ways to properly (cleanly) stop a job once it's started based on some conditions of the git repository. While searching for this, I came across several support questions with a similar answer: "use `circleci step halt`". Apparently this stops the job and marks it as successful.
The thing is, I can't seem to find any official documentation. Would be a great addition since it's very useful!