Change communication to slack

[ashkan-software]

If we want the users to use Slack, we should change the contents on FAQ on the website as well (currently, it shows StackOverflow)

[ashkan-software]

It is also on the Contact Us page:
https://flow-project.github.io/contact.html

[eugenevinitsky]

Ah, great point.

[eugenevinitsky]

In PR #34

[ashkan-software]

@eugenevinitsky don't forget the FAQ on Get Started Page