Add page on common issues causing apps to not work

[timjrobinson]

One thing users should try is doing "curl localhost:8080" to see if that works and determine if it's a problem with their app or the proxy.

• [ ] Use curl to see if it's a proxy problem
• [ ] Ensure app is running on 0.0.0.0. (localhost and 127.x.x.x will not work).