Document how to get a list of builtins for common compatible shells

[abathur]

Once I add support for overriding the resolved builtins in #10, I think it'd be helpful to document/list how a user might generate an appropriate list.

For now this is mostly a braindump of ways I'm aware of that might yield an automatic list of builtins for some shells for refining later.

• enable: bash dgsh zsh
• builtin: ksh
• builtin --names: fish
• builtins: csh, tcsh
• compgen -b or compgen -A builtin: bash