README: canonical www.cratis.io links and a definition-first opening #35

The README should open with one plain sentence that says what this repository is, who it is for and what it does, and link Cratis documentation on the canonical https://www.cratis.io host (the apex https://cratis.io redirects). Part of Cratis/SEO#12 and Cratis/SEO#26.

Status

State: Done
Branch: (#36)
Scope: README www links + definition-first (r19)
Delivered: Nothing yet.
Remaining: To be confirmed.
Follow-ups: None.