Add README to teach contributor how to run the server.

Dear Manske, great tool. I'd like to contribute to fix the issue that it doesn't render the X-axis very well. But coming from a development background mostly with NodeJS, it's greek to me in PHP. Could you kindly add some README to show me how to start the server for wikidata-todo repostiory?

User:Xinbenlv (for some reason my bitbucket account doesn't work...)