Update README for metafield

[ntn0de]

shopify.metafield
  .create({
    key: 'warehouse',
    value: 25,
    value_type: 'integer',
    namespace: 'inventory',
    owner_resource: 'product',
    owner_id: 632910392
  })
  .then(
    (metafield) => console.log(metafield),
    (err) => console.error(err)
  );

value_type causes error on new API, use type instead