Document possible limitations with CmdRunner module

[senorprogrammer]

Per the examples raised in this issue, CmdRunner cannot run everything. Some documentation and tips for trouble-shooting would be helpful.

[tnwei]

Arrived here from the aforementioned issue. I think referring to the fact that CmdRunner relies on Golang's exec.Command() is a good piece of info for troubleshooting. It does not support shell expansion, which I believe would be a common gotcha.

Off-topic, but wonder if it is possible to pipe the command output directly to stdout so that more info is available for debugging other than the exit statuses provided by Golang.