trace: document the Stackdriver exporter
trace: document the Stackdriver exporter

Without the exporter note, user don't know how to export
directly to Stackdriver Trace.

Change-Id: I9772044ba15a797e2661e50a62eb7f56a3a852a4
Reviewed-on: https://code-review.googlesource.com/25370
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jean de Klerk <605b93664484fed1e671a07bff68da39e73d0174@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go