Add Slack badge to README.md
Add Slack badge to README.md

colmena/colmena,colmena/colmena-cms,colmena/colmena,colmena/colmena,beeman/loopback-angular-admin,colmena/colmena-cms,colmena/colmena-cms,beeman/loopback-angular-admin,beeman/loopback-angular-admin