Adds link to Slack inviter (#6584)
Adds link to Slack inviter (#6584)

This badge was linking straight to Slack, which requires an invite. Now it links to the inviter app on Heroku.
konnecteam/sequelize,Alexsey/sequelize,sequelize/sequelize,Verdier/sequelize,seshness/sequelize,yjwong/sequelize,grimurd/sequelize,yonjah/sequelize,PsiXdev/sequelize,inDream/sequelize,sorin/sequelize,Americas/sequelize,darwinserrano/sequelize,StefanoDalpiaz/sequelize,pola88/sequelize,cbauerme/sequelize,majorleaguesoccer/sequelize,alekbarszczewski/sequelize,inDream/sequelize,mikeringrose/sequelize,sequelize/sequelize,overlookmotel/sequelize,konnecteam/sequelize,Americas/sequelize,jayprakash1/sequelize,abegines/sequelize,overlookmotel/sequelize,nemisj/sequelize,sequelize/sequelize,PhinCo/sequelize,jayprakash1/sequelize,tornillo/sequelize,gorangajic/sequelize,Wsiegenthaler/sequelize,p0vidl0/sequelize,gintsgints/sequelize