Add Slack Community

[zenopopovici]

Add Slack community link to the readme.

[loverajoel]

Nice, we have a designer in the team, probably can help us with the landing page design before the official release

[zenopopovici]

@loverajoel Cool. I've transfered Heroku ownership. You'll have to accept it to complete.

[zenopopovici]

Also a custom domain can be attached.

[loverajoel]

I will set slack.jstips.co