message: add build script and x0 config
diff: "description": "Primer react components",
  "main": "index.js",
  "scripts": {
  - "start": "x0 dev src/Index.js -op 8888"
  + "start": "x0 dev src/Index.js -op 8888",
  + "build": "x0 build src/Index.js --out-dir site"
  },
  "repository": {
  "type": "git",
  "react": "^16.2.0",
  "styled-components": "^3.1.6",
  "styled-system": "^2.0.2"
  + },
  + "x0": {
  + "cssLibrary": "styled-components",
  + "basename": "/primer-react"
  }
  }
  