Update feature requests doc

Summary:
This mentions "like GitHub", but we purged all the issues and no longer accept them.

Generally, feature requests should be coming to the upstream only nowadays.

Also, don't overpromise IRC.

Test Plan: Read documentation.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D11777