Add a method for generating sitemap url

[PikachuEXE]

I need to generate the URL for robot.txt
I am doing it this way now:

module SeoHelper
  SITEMAP_INDEX_FILE_NAME = 'sitemap.xml.gz'

  def uploaded_sitemap_url
    [
      SitemapGenerator::Sitemap.sitemaps_host,
      SitemapGenerator::Sitemap.sitemaps_path,
      SITEMAP_INDEX_FILE_NAME,
    ].join
  end
end

But due to #124 it might be dependent on the option set
So it might be better to have a method to generate it

[kjvarga]

SitemapGenerator::Sitemap.sitemap_index_url is what you want.

[PikachuEXE]

Thx, maybe it can be added to README?