wishlist: Mention command line interface

[hackerb9]

The README and documentation do not mention a command line interface or any sort of reference implementation. This is something that people do look for and so it would be good to document it clearly so people don't have to do an exhaustive search to know that none exists.

Please consider adding to the README something like,

If you are looking for a command line interface (CLI) or reference implementation, you've come to the wrong place. Pyotp is only a library which is used by other projects that provide a user interface, for example, 3ncrypt0rQR or pyotp_cli.

Thank you.