Document 'google-cloud-trace' and BackgroundThreadTransport (#262)

Author: stewartreichling

README.rst

@@ -136,6 +136,31 @@ This example shows how to report the traces to Stackdriver Trace:
         project_id='your_cloud_project')
     tracer = tracer_module.Tracer(exporter=exporter)
 
+StackdriverExporter requires the google-cloud-trace package. Install
+google-cloud-trace using `pip`_ or `pipenv`_:
+
+::
+
+    pip install google-cloud-trace
+    pipenv install google-cloud-trace
+
+By default, traces are exported synchronously, which introduces latency during
+your code's execution. To avoid blocking code execution, you can initialize
+your exporter to use a background thread.
+
+This example shows how to configure Census to use a background thread:
+
+.. code:: python
+
+    from opencensus.trace.exporters import stackdriver_exporter
+    from opencensus.trace import tracer as tracer_module
+    from opencensus.trace.exporters.transports.background_thread \
+        import BackgroundThreadTransport
+
+    exporter = stackdriver_exporter.StackdriverExporter(
+        project_id='your_cloud_project', transport=BackgroundThreadTransport)
+    tracer = tracer_module.Tracer(exporter=exporter)
+
 Propagators
 ~~~~~~~~~~~