update the doc with TraceContextPropagator (#309)

* update the doc with TraceContextPropagator

* make a real working sample

* put TraceContextPropagator as a separate sample

Author: reyang

README.rst

@@ -169,8 +169,8 @@ Propagators
 ~~~~~~~~~~~
 
 You can specify the propagator type for serializing and deserializing the
-``SpanContext`` and its headers. There are currently two built in propagators:
-``GoogleCloudFormatPropagator`` and ``TextFormatPropagator``.
+``SpanContext`` and its headers. There are currently three built in propagators:
+``GoogleCloudFormatPropagator``, ``TextFormatPropagator`` and ``TraceContextPropagator``.
 
 This example shows how to use the ``GoogleCloudFormatPropagator``:
 
@@ -186,6 +186,23 @@ This example shows how to use the ``GoogleCloudFormatPropagator``:
     # Serialize
     header = propagator.to_header(span_context)
 
+This example shows how to use the ``TraceContextPropagator``:
+
+.. code:: python
+
+    import requests
+
+    from opencensus.trace import config_integration
+    from opencensus.trace.propagation.trace_context_http_header_format import TraceContextPropagator
+    from opencensus.trace.tracer import Tracer
+
+    config_integration.trace_integrations(['httplib'])
+    tracer = Tracer(propagator = TraceContextPropagator())
+
+    with tracer.span(name = 'parent'):
+        with tracer.span(name = 'child'):
+            response = requests.get('http://localhost:5000')
+
 Blacklist Paths
 ~~~~~~~~~~~~~~~