Cross-reference ECS reference deployment
This links back out to GitHub to show the full ECS reference deployment.
Alternatively, we could create a new deployment guide in our docs site
with the contents of the README.
I think the rest of the AWS content is otherwise worth keeping (the RDS
and S3 stuff) so I didn't want to replace it wholesale.
Test Plan: none
Reviewers: catherinewu, dgibson, johann
Reviewed By: dgibson
Subscribers: sashank, yuhan
Differential Revision: https://dagster.phacility.com/D8491