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

externals/includes/README

@@ -1,3 +1,3 @@
 This directory is added to the PHP include path. You can symlink things here
 if you have control over directory setup but not over PHP configuration (for
-instance, in Homebrew).
+instance, in Homebrew).