feature: Issue warning if pymdownx.superfences is not enabled

[pawamoy]

Is your feature request related to a problem? Please describe.

Several users, including myself, couldn't understand why code blocks weren't rendered. It was because the pymdownx.superfences was not enabled. The readme does not really explain that.

Describe the solution you'd like

Add a note to the readme, and when using as a plugin, detect when pymdownx superfences is not enabled, to issue an error.

Describe alternatives you've considered

Just docs. But we can do better.

Additional context

#37