added doc for asset hosts
added doc for asset hosts

rstacruz/sinatra-assetpack,jwhitcraft/sinatra-assetpack,rstacruz/sinatra-assetpack,rstacruz/sinatra-assetpack,jwhitcraft/sinatra-assetpack,flywithmemsl/sinatra-assetpack,jwhitcraft/sinatra-assetpack,flywithmemsl/sinatra-assetpack,flywithmemsl/sinatra-assetpack