Unit Test instructions in README fail (missing semistandard dependency call-out)

[beccasaurus]

If you follow the "Running the tests" instructions for any product, they fail.

We do not explicitly tell the user that semistandard is required and this has resulted in users not being able to run npm test nor easily debug how to fix their testing environment.

Recommended: call-out semistandard as a requirement on the prerequisites page

https://github.com/GoogleCloudPlatform/nodejs-docs-samples/blob/master/README.md#prerequisites