Document podman compatability

Seems to require podman v2.x ? https://github.com/common-workflow-language/cwltool/pull/1343#issuecomment-700370326

Might require being run as root? https://cwl.discourse.group/t/cwl-runner-and-podman/210/2 (maybe that can be fixed another way)

Bonus: autodetect podman vs docker; have a command line option to override