Split Stackdriver exporter to its own repo (#724)

Vendor-specific exporters will be maintained at the
census-ecosystem organization. This allows us to keep
the core libraries minimal and vendors to have all
autonomy on the vendor-specific exporter implementations.

Fixes #720.

README.md

@@ -176,7 +176,7 @@ A screenshot of the CPU profile from the program above:
 [new-replace-ex]: https://godoc.org/go.opencensus.io/tag#example-NewMap--Replace
 
 [exporter-prom]: https://godoc.org/go.opencensus.io/exporter/prometheus
-[exporter-stackdriver]: https://godoc.org/go.opencensus.io/exporter/stackdriver
+[exporter-stackdriver]: https://godoc.org/contrib.go.opencensus.io/exporter/stackdriver
 [exporter-zipkin]: https://godoc.org/go.opencensus.io/exporter/zipkin
 [exporter-jaeger]: https://godoc.org/go.opencensus.io/exporter/jaeger
 [exporter-xray]: https://github.com/census-instrumentation/opencensus-go-exporter-aws

exporter/stackdriver/stackdriver.go

@@ -12,13 +12,9 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// Package stackdriver contains the OpenCensus exporters for
-// Stackdriver Monitoring and Stackdriver Tracing.
+// Package stackdriver has moved.
 //
-// Please note that the Stackdriver exporter is currently experimental.
-//
-// The package uses Application Default Credentials to authenticate.  See
-// https://developers.google.com/identity/protocols/application-default-credentials
+// Deprecated: Use contrib.go.opencensus.io/exporter/stackdriver instead.
 package stackdriver // import "go.opencensus.io/exporter/stackdriver"
 
 import (