Document in FAQ how to create Twitter Player <iframe> cards

See https://twitter.com/cvanw/status/974068622468423680

And Twitter's Docs: https://developer.twitter.com/en/docs/tweets/optimize-with-cards/overview/player-card

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(Also, to do this for https://webvr.rocks, there's this issue for writing a script to generate the markup for various demos: #158)
