Update "Test Metadata" documentation to include PHP

[gena01]

It seems that https://circleci.com/docs/test-metadata is missing PHP.

PHPUnit can generate junit xml files as an option via "–log-junit ". See documentation here: https://phpunit.de/manual/current/en/textui.html

[drazisil]

Docs have been updated, thank you very much for sharing!