Documentation: add an "Is Rust used in production?" FAQ

[kud1ing]

Mention

• http://labs.umbrella.com/2013/10/04/zeromq-helping-us-block-malicious-domains/
• https://www.skylight.io/

[flaper87]

I expect this list to grow a lot in the future and it's probably better to have it in the website. See Python's example: https://www.python.org/about/success/