Add heroku instructions and default settings.

[Jeff Dutil]

# pick a place safe to write the files
SitemapGenerator::Sitemap.public_path = 'tmp/'
# store on S3 using Fog
SitemapGenerator::Sitemap.adapter = SitemapGenerator::S3Adapter.new({aws_access_key_id:     Spree::Config[:s3_access_key], 
                                                                     aws_secret_access_key: Spree::Config[:s3_secret], 
                                                                     fog_provider:          'AWS',
                                                                     fog_directory: Spree::Config[:s3_bucket]})

# inform the map cross-linking where to find the other maps
SitemapGenerator::Sitemap.sitemaps_host = "http://#{Spree::Config[:s3_bucket]}.s3.amazonaws.com/"
# pick a namespace within your bucket to organize your maps
SitemapGenerator::Sitemap.sitemaps_path = 'sitemaps/'