Update README.md for v3.0.0

[marijnvanwezel]

This pull request updates the documentation to reflect the breaking changes in v3.

It would be good to release master as a tag (say 3.0.1), or cherry-pick this commit along with 1003ca1 and 1d16719 and release that instead, as soon as possible. Namely, the current documentation on hexdocs is incorrect. Following it gives the somewhat vague error:

* (Mix) Could not start application myapp: MyApp.Application.start(:normal, []) returned an error: shutdown: failed to start child: MyApp.Telemetry
    ** (EXIT) an exception was raised:
        ** (ArgumentError) The module Guardian.DB.Token.SweeperServer was given as a child to a supervisor but it does not exist