Add shell target to documentation

[jheiselman]

Please add which shell you are targeting to the documentation. Bash/ksh/POSIX-strict. Would help a lot to know if this can be used in environments that require some of those lesser used shells for automation.

[Luavis]

It is developed to target Bash currently. I'll add it to the documentation. From your suggestion, I'll plan to support targeting other shell(include windows powershell). Thanks.