[doc]: Update README.md and 'zinit help' with new subcommands (plugins, snippets)

[mojolo]

Documentation Link

https://github.com/zdharma-continuum/zinit/blob/main/README.md

Problem

Firstly, many thanks to @vladdoster, @pschmitt ,et al, for taking over the zinit project. Don't know what I'd do without it!

I was at whit's end trying to figure out how to get zinit to list my installed plug-ins and snippets. Kept trying zinit list, zinit loaded, zinit ls. Read zinit help and the README.md over and over. But kept getting the following error:

ERROR: Unknown subcommand: {`list` , 'loaded', 'ls'} (it should be one of, e.g.: `load`, `snippet`, `update`, `delete`, …, e.g.: `zinit load username/reponame`) or a for-based command body (i.e.: e.g.: `zinit …ice-spec… for …(plugin or snippet) ID-1 ID-2 ⋯⋯…`). See `help` for a more detailed usage information and the list of the subcommands.

Finally found #497 and voila: zinit plugins and zinit snippets did the trick.

Suggested improvement

Please update README.md and zinit help to reflect the changes in #497 so other users don't face the same issue.

Ty!