Adjust CloudFront configuration instructions to allow POST

Summary: Ref T8293. We may submit POST requests to the alternate file domain, and CloudFront should be configured to route them.

Test Plan: See T8293.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8293

Differential Revision: https://secure.phabricator.com/D12973