Add comments that explain where $STACK comes from
Add comments that explain where $STACK comes from

Genius/heroku-buildpack-ruby,andycroll/heroku-buildpack-jekyll,makifund/heroku-buildpack-ruby,donnykurnia/heroku-buildpack-ruby,jonseaberg/heroku-buildpack-turbo-sprockets,pilotcreative/heroku-buildpack-ruby,makifund/heroku-buildpack-ruby,pilotcreative/heroku-buildpack-ruby,mattmanning/heroku-buildpack-ruby-jekyll,orenyk/heroku-buildpack-ruby,Scalingo/ruby-buildpack,pkgr/heroku-buildpack-ruby,instacart/heroku-buildpack-ruby,orenyk/heroku-buildpack-ruby,instacart/heroku-buildpack-ruby,ericboehs/heroku-buildpack-ruby,Genius/heroku-buildpack-ruby,ericboehs/heroku-buildpack-ruby,donnykurnia/heroku-buildpack-ruby,andycroll/heroku-buildpack-jekyll,ruby/heroku-buildpack-www-ruby-lang,heroku/heroku-buildpack-ruby,zunda/heroku-buildpack-ruby,pkgr/heroku-buildpack-ruby,heroku/heroku-buildpack-ruby,zunda/heroku-buildpack-ruby,brightbytes/heroku-buildpack-ruby-without-assets-precompile,mattmanning/heroku-buildpack-ruby-jekyll,jonseaberg/heroku-buildpack-turbo-sprockets,Scalingo/ruby-buildpack,brightbytes/heroku-buildpack-ruby-without-assets-precompile