Add coveralls badge to README
Add coveralls badge to README

AndrewCarterUK/Slim,dopesong/Slim,akrabat/Slim,iinux/Slim,jaapverloop/Slim,mnapoli/Slim,iinux/Slim,RealSelf/Slim,somjit2514/basic,Sam-Burns/Slim,samsonasik/Slim,foxyantho/Slim,iinux/Slim,designermonkey/Slim,slimphp/Slim,opengeek/Slim,juliangut/Slim,feryardiant/slim,JoeBengalen/Slim