Add a troubleshooting guide

[findleyr]

We've had several issues related to interactions between the SDK and other clients or hosts.

Sometimes, they are our bug, sometimes they are problems with the other end of the connection (and sometimes the spec is just unclear!).

For these cases, it would be helpful to have a troubleshooting guide describing how to debug these interactions, for example by using the inspector, or adding a LoggingTransport.

[djoreilly]

I found httpdump most useful. Dump everything on loopback with json pretty-printed:

$ sudo httpdump --device lo --level all --pretty