Update CONTRIBUTING.md
Update CONTRIBUTING.md

Added --acl-public to copy
pierredup/packager-buildpack-php,CHH/heroku-buildpack-php,nsg/dokku-buildpack-picocms,stof/heroku-buildpack-php,Jonatasmz/php-buildpack,pierredup/packager-buildpack-php,ryosuzuki/php-buildpack,Scalingo/php-buildpack,cachethq/cachet-buildpack,CHH/heroku-buildpack-php,CHH/heroku-buildpack-php,AlphaStaxLLC/heroku-buildpack-php-custom,briankilgore/heroku-buildpack-php-prod,penciu/apache,AlphaStaxLLC/heroku-buildpack-php-custom,AlphaStaxLLC/heroku-buildpack-php-custom,Scalingo/php-buildpack,Scalingo/php-buildpack,jstacoder/heroku-buildpack-php-1,OKAMca/heroku-buildpack-php,stof/heroku-buildpack-php,OKAMca/heroku-buildpack-php,stof/heroku-buildpack-php,ryosuzuki/php-buildpack,penciu/apache,mraymond/heroku-buildpack-php,bobmaerten/dokku-buildpack-php,OKAMca/heroku-buildpack-php,Jonatasmz/php-buildpack,nsg/dokku-buildpack-picocms,penciu/apache,jstacoder/heroku-buildpack-php-1,mraymond/heroku-buildpack-php,bobmaerten/dokku-buildpack-php