Add a newline to externals/includes/README

Summary: For consistency, add a trailing newline to externals/includes/README. See D9940.

Test Plan: N/A

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9950