trace: document the Stackdriver exporter
Without the exporter note, user don't know how to export
directly to Stackdriver Trace.
// See the License for the specific language governing permissions and
// limitations under the License.
-// This package is OBSOLETE. See https://godoc.org/go.opencensus.io/trace.
+// This package is OBSOLETE. See https://godoc.org/go.opencensus.io/trace; and use
+// OpenCensus Stackdriver exporter, https://godoc.org/go.opencensus.io/exporter/stackdriver.
//
// Package trace is a Google Stackdriver Trace library.
//
