Never try to run README as a commit hook

Summary:
Fixes T4960. Users chmod +x this, and then bash chokes on it.

Phabricator "owns" this file anyway, so there is no real ambiguity here: this should never be a hook script.

Test Plan:

• Did chmod +x README.
• Made a commit.
• Added z.sh, got blocked.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T4960

Differential Revision: https://secure.phabricator.com/D8981