Improve user-facing documentation

[kasparsd]

Is your enhancement related to a problem? Please describe.

The current README on WP.org for this plugin is very sparse and could do a lot better at guiding users through the whole setup process of the two-factor plugin and also link to known limitations.

Proposed Solution

• Describe the setup -- how users have to individually configure the second factor in their admin profile.
• Describe how the plugin has no settings yet. Link to the relevant issue.
• Describe how to enable WebAuthn using the other plugin.
• Consider linking to https://developer.wordpress.org/advanced-administration/security/mfa/ for general context.

Designs

No response

Describe alternatives you've considered

No response

Please confirm that you have searched existing issues in this repository.

Yes