Please add sub-command example to the documentation

[wnortje]

HI

In this Reddit thread, https://old.reddit.com/r/Common_Lisp/comments/m7gjno/writing_small_cli_programs_in_common_lisp_steve/grdqq1j/, you provide an example for using Adopt in a sub-command style program. Even if it is not designed for it, using Adopt for sub-command parsing is clean and straightforward. Please add the example to the documentation.