Add json-schema (PHP) to list of who uses the test suite.

I added [json-schema (PHP)](https://github.com/justinrainbow/json-schema) to the list of who uses this test suite in the README.md file. I thought that adding a composer.json file would help PHP projects include this project as a repository dependency for Composer to resolve, but I later found out that it wasn't necessary so I deleted it. Please ignore the two commits that add and then delete composer.json. Thanks!