Looks like there's a good one here:

https://github.com/mvdan/sh/blob/master/cmd/shfmt/shfmt.1.scd

PR's welcome. This should be pretty easy, you can follow the example of BlackStep and ClangFormatStep. See #672 for more details on how they work.