[test-infra] Mention installing html5lib to run the tests in the README.md (#857)

Fix https://github.com/w3c/web-platform-tests/issues/543

This is the trap for beginners. :cry:

View on GitHub: https://github.com/w3c/web-platform-tests/pull/857

Received on Saturday, 12 April 2014 05:46:38 UTC