added missing install instruction
added missing install instruction

lvnhmd/Nodemailer,Jonekee/Nodemailer,RIAEvangelist/Nodemailer,dkoleary88/Nodemailer,motdotla/Nodemailer,catalinmiron/Nodemailer,thirdiron/Nodemailer,SparkPost/Nodemailer,VAS/Nodemailer,Andrey-Pavlov/Nodemailer,eahefnawy/Nodemailer,mhtranbn/Nodemailer,hanan198501/Nodemailer,leeric92/Nodemailer,elbakai/Nodemailer,cvibhagool/Nodemailer,CristinaFoltea/Nodemailer,andris9/Nodemailer,ferreiro/Nodemailer,niftylettuce/Nodemailer,songshuang00/Nodemailer,Feitianyuan/Nodemailer,pandeysoni/Nodemailer