add doc on using Rack middleware to README
add doc on using Rack middleware to README

nottombrown/old-sinatra,nicoayala/sinatra,sethuster/sinatra,gferguson-gd/sinatra,ganmacs/sinatra,defsprite/sinatra,autopp/sinatra,ksw2599/sinatra,davydovanton/sinatra,twanies/sinatra,eileencodes/sinatra,guidohcosta/sinatra,JonMidhir/sinatra,ABaldwinHunter/sinatra-classic,ganmacs/sinatra,miyazawatomoka/sinatra,ktw1222/sinatra,JonRowe/sinatra,burningTyger/sinatra,emiliocorpus/sinatra,jenalgit/sinatra,QLGu/sinatra,d-bot/sinatra,porras/sinatra,ryanshaw/sinatra,davydovanton/sinatra,mwpastore/sinatra,kumji/sinatra,trexnix/sinatra,RosenDimov/sinatra,kumji/sinatra,kyontan/sinatra,JonRowe/sinatra,ABaldwinHunter/sinatra-classic,sinatra/sinatra,DoraALin/sinatra,JonMidhir/sinatra,Callygraphy/morrissey,dariubs/sinatra,burningTyger/sinatra,blambeau/sinatra,sethuster/sinatra,DoraALin/sinatra,howtolearntocode/sinatra,Rayheng/sinatra,autopp/sinatra,twanies/sinatra,dariubs/sinatra,jenalgit/sinatra,ryanshaw/sinatra,defsprite/sinatra,laplaceliu/sinatra,burningTyger/sinatra,JonMidhir/sinatra,nju520/sinatra,eileencodes/sinatra,frechei/sinatra,midorineko/sinatra,kyontan/sinatra,gferguson-gd/sinatra,porras/sinatra,lastcat/sinatra,siberianbear/sinatra,jkowens/sinatra,frechei/sinatra,QLGu/sinatra,mwpastore/sinatra,nottombrown/old-sinatra,Rayheng/sinatra,jkowens/sinatra,wzheng1/sinatra,kgrz/sinatra,trexnix/sinatra,laplaceliu/sinatra,nicoayala/sinatra,marocchino/sinatra,mwpastore/sinatra,miyazawatomoka/sinatra,ktw1222/sinatra,guidohcosta/sinatra,cyberid41/sinatra,midorineko/sinatra,emiliocorpus/sinatra,d-bot/sinatra,wzheng1/sinatra,cyberid41/sinatra,nju520/sinatra,lastcat/sinatra,ksw2599/sinatra,bmizerany/sinatra,RosenDimov/sinatra,howtolearntocode/sinatra,siberianbear/sinatra,Wirachmat/sinatra,jkowens/sinatra,sinatra/sinatra,Wirachmat/sinatra