Readme update on module.exports sitemap configuration for VuePress 1.0

[anibalsanchez]

Hi,

I'm finding a different way to configure the plugin. I think this is currently the supported method in VuePress 1.0.*

  plugins: [
    [
      'sitemap',
      {
        hostname: 'https://www.extly.com/docs',
      },
    ],
  ],

[ekoeryanto]

AFAIK, both are supported except they change it, I'll check it out.

Thanks for the PR anyway

[ekoeryanto]

I found they still support the object style plugin