jaeger: Usage README Update for opentelemetry-ext-jaeger (open-telemetry#459)

Usage docs for opentelemetry-ext-jaeger need to be updated after the change to `TracerSource` with v0.4. Looks like it was partially updated already.

Users following the usage docs will currently run into this error:
`AttributeError: 'Tracer' object has no attribute 'add_span_processor'`

Author: toumorokoshi

ext/opentelemetry-ext-jaeger/README.rst

@@ -56,7 +56,7 @@ gRPC is still not supported by this implementation.
     span_processor = BatchExportSpanProcessor(jaeger_exporter)
 
     # add to the tracer
-    tracer.add_span_processor(span_processor)
+    trace.tracer_provider().add_span_processor(span_processor)
 
     with tracer.start_as_current_span('foo'):
         print('Hello world!')