Create GoDoc example for using rego.Tracer

[tsandall]

We don't have an example that shows how to enable tracing--but this comes up often enough that it would be good to have something to link to. We should at least have a GoDoc example that shows how to run query evaluation with tracing enabled.

Something like: https://github.com/open-policy-agent/opa/blob/master/rego/example_test.go#L20 but with:

buf := topdown.NewBufferTracer()

r := rego.New(
  // other args
  rego.Tracer(buf),
)

// call eval

topdown.PrettyTrace(os.Stdout, *buf)