Add a contribute.json

[openjck]

Mozilla says that this file should be served up for all websites.

https://infosec.mozilla.org/guidelines/web_security#contributejson

[pdehaan]

Rough stab:

{
  "name": "ensemble",
  "description": "Ensemble is the platform that powers the Firefox Public Data Report (https://data.firefox.com), a weekly public report on the activity, behavior, and hardware configuration of Firefox Desktop users.",
  "repository": {
    "url": "https://github.com/mozilla/ensemble",
    "license": "MPL-2.0"
  },
  "keywords": [
    "D3",
    "JavaScript",
    "Node",
    "React"
  ],
  "bugs": {
    "list": "https://github.com/mozilla/ensemble/issues",
    "report": "https://github.com/mozilla/ensemble/issues/new"
  },
  "urls": {
    "prod": "https://data.firefox.com/"
  }
}

Would also be nice to add dev/stage URLs, links to Circle/Travis CI builds, links to any docs, etc.