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