No list of commands in the --help output?

Feels a bit silly but it seems like you missed the list of actual commands from the output you get when you run "s4cmd --help". It even misses in the one-line synopsis to tell you there is something else than just options needed:
```
$ s4cmd --help
Usage: s4cmd [options]

Super S3 command line tool. Version 2.1.0
[long list of options here]
```

I know the commands are listed on the web and I know s3cmd commands but if this tool is to be standing on its own legs and not just be "like s3cmd but with.." it should probably not assume so much familiarity with s3cmd as to skip the actual commands needed to put/del/sync files.

Start at the CLI entry point that handles `s4cmd --help` and run the command to inspect how its usage text is assembled. Update the synopsis and help output to list the available commands, including operations such as put, del, and sync; done means a new user can discover these commands without relying on s3cmd documentation.

Written by the indexing model from the issue text.