Add new Slack icon to README
Add new Slack icon to README

[#136438447]

matiasinsaurralde/tyk-buildpack,rakutentech/binary-buildpack,rakutentech/binary-buildpack,rakutentech/binary-buildpack,cloudfoundry/binary-buildpack,matiasinsaurralde/tyk-buildpack,cloudfoundry/binary-buildpack,matiasinsaurralde/tyk-buildpack,matiasinsaurralde/tyk-buildpack,matiasinsaurralde/tyk-buildpack,matiasinsaurralde/tyk-buildpack,cloudfoundry/binary-buildpack,matiasinsaurralde/tyk-buildpack