Now that we have a test.bash script that does comprehensive testing,
add instructions to CONTRIBUTING.md for how to install the hook.
We use a pre-push hook, rather than a pre-commit hook (which is more common)
since we're more interested in making sure the changes are correct just prior
to a user sending out code for review.
It is common for a user to continually add new files and/or update the commit
message. It would be annoying running the tests for every one of those operations.
Change-Id: I084a74fdb7f601d8075b3e349c7ee854b3b9394b
Reviewed-on: https://go-review.googlesource.com/128364
Reviewed-by: Herbie Ong <herbie@google.com>