Adds guides for running PHP frameworks/PHP products

[tmatsuo]

The reason I wanted to add this info to our documentation is due to the inefficiency on reviewing the document PR on this repo, CircleCI will run our full tests for each edit. Also If we add it, I want to also add tests for that because such information can be easily become stale.

I still want to add such documentation and tests to other places; docs on https://cloud.google.com/php and tests in php-docs-samples.

[bshaffer]

agreed! The doc site is a better place for this, as long as we link to the docs from the README

[bshaffer]

This is being tracked elsewhere, and doesn't really belong in this repo.