add --dry-run to list of unsupported pip-install options in README

[`--dry-run`](https://pip.pypa.io/en/stable/cli/pip_install/#cmdoption-dry-run) was added in [pip v22.2](https://pip.pypa.io/en/stable/news/#v22-2). Since the purpose of smuggling a package is to load it for immediate use, onion comments cannot contain `pip install` arguments that would cause the package to not actually be installed — `-h`/`--help` and `-v`/`--version` are disallowed for the same reason.