Change IRC to Slack on README
Change IRC to Slack on README

mferenca/HMS-ecommerce,eduNEXT/edunext-ecommerce,eduNEXT/edunext-ecommerce,edx/ecommerce,edx/ecommerce,edx/ecommerce,mferenca/HMS-ecommerce,eduNEXT/edunext-ecommerce,edx/ecommerce,mferenca/HMS-ecommerce,eduNEXT/edunext-ecommerce