Document ENV values for CI

[hayes0724]

Values in src/env/packer-env.schema.js should be documented for CI usage:

const commonPaths = require('../config/common-paths');

module.exports = {
  ...commonPaths,

  // The environment variable key which contains the name of the environment
  // Packer is running in
  'env.keys.name': 'PACKER_ENV',

  // The environment variable key which contains the myshopify.com URL to your
  // Shopify store
  'env.keys.store': 'PACKER_STORE',

  // The environment variable key which contains the API password generated from
  // a Private App
  'env.keys.password': 'PACKER_PASSWORD',

  // The environment variable key which contains the ID of the theme you wish to
  // upload files to
  'env.keys.id': 'PACKER_ID',

  // The environment variable key which contains a list of file patterns to
  // ignore, with each list item separated by ':'
  'env.keys.ignore': 'PACKER_IGNORE',

  // The environment variable key which contains a list of file patterns to
  // allow deployment to live themes
  'env.keys.live': 'PACKER_LIVE',
};

[hayes0724]

Done: https://hayes0724.github.io/shopify-packer/docs/config/settings/