Fix the airline demo README

Summary:
The airline demo README had a dead link. This removes it, explains what the airline demo is, and adds a warning.

https://github.com/dagster-io/dagster/issues/4205

Test Plan: none

Reviewers: max, schrockn, yuhan

Reviewed By: yuhan

Differential Revision: https://dagster.phacility.com/D8062