Add a newline to `externals/includes/README`

Summary: For consistency, add a trailing newline to `externals/includes/README`.

Test Plan: N/A

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9940