README: Clarify what the special `not_found` and `error` handlers do
README: Clarify what the special `not_found` and `error` handlers do

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