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