added contribution guideline
added contribution guideline

fixes https://github.com/linnovate/mean/issues/1392

fkiller/mean,tedtkang/meanTestApp,mstackpo/MattsMeanApp,rutys/pictionary,kevinzli/meanforms,minzen/u-qasar-mean,cyberTrebuchet/this_is_forOM,jackrosenhauer/nodebot-mean,wearska/mean-mono,misterdavemeister/meanio,vildantursic/vildan-mean,GeminiLegend/myRelephant265,NenadP/blocks,rachelleannmorales/meanApp,fkiller/harvard,rutys/pictionary,bbayo4/Businness-Application-using-MEAN.IO-Stack,VikramTiwari/mean,kevinzli/meanforms,jun-oka/bee-news-jp-client-1,yosifgenchev/iPlayIt,ascreven/mean.io,fkiller/harvard,vildantursic/vildan-mean,vikram-bagga/zummy,komarudin02/Public,09saurabh09/vocal,anthony-ism/funari,idhere703/meanSample,VikramTiwari/mean,yjkim1701/cmt-mgr,tmrotz/brad,ascreven/mean.io,smadjid/CoReaDa,Peretus/test-mean,Peretus/test-mean,rjVapes/mean,Sharmaz/mean,tedtkang/meanTestApp,komarudin02/Public,jenavarro/AllRules,tmrotz/brad,tmrotz/brad,rpalacios/preach-lander,spacetag/TechEx_Josiah,mstackpo/MattsMeanApp,yosifgenchev/iPlayIt,Cneubauern/Test,smadjid/CoReaDa,misterdavemeister/meanio,cyberTrebuchet/this_is_forOM,yjkim1701/cmt-mgr,joshsilverman/me,misterdavemeister/meanio,idhere703/meanSample,jun-oka/bee-news-jp-client-1,jenavarro/AllRules,smadjid/CoReaDa,rutys/pictionary,minzen/u-qasar-mean,Cneubauern/Test,vikram-bagga/zummy,fkiller/mean,rachelleannmorales/meanApp,jackrosenhauer/nodebot-mean,oritpersik/admin-dashboard,rjVapes/mean,anthony-ism/funari,goblortikus/stroop,NenadP/blocks,ryanlhunt/sim_ventory,oritpersik/admin-dashboard,joshsilverman/me,ryanlhunt/sim_ventory,bbayo4/Businness-Application-using-MEAN.IO-Stack,spacetag/TechEx_Josiah,Sharmaz/mean,GeminiLegend/myRelephant265,wearska/mean-mono,09saurabh09/vocal,rpalacios/preach-lander,goblortikus/stroop