[test-infra] [readme] Make the readme intro better. (#1720)

This is a slightly re-worded copy from @jgraham summary in https://lists.mozilla.org/pipermail/dev-platform/2015-March/009167.html

View on GitHub: https://github.com/w3c/web-platform-tests/pull/1720

Received on Monday, 6 April 2015 12:49:46 UTC