Mention that ports have to be open on firewalls for tutorials to work #24

Fn uses 8080 which is usually open, but Flow uses 8081 which most firewalls don't open by default. The UI also uses ports that are not open by default.

Some users have encountered hard-to-diagnose trouble because the firewall was blocking communication to flow or the ui when everything is running on a single host.

We have to mention that the appropriate ports have to be open on the firewall. This affects:

• Flow 101 and 102
• Flow Saga
• ...?