update zipkin example README.md

With instructions on how to use docker-compose to run the example.

Signed-off-by: Andrew Hsu <[email protected]>

example/zipkin/README.md

@@ -1,16 +1,22 @@
 # Zipkin Exporter Example
 
-Sends spans to zipkin collector.
+Send an example span to a [Zipkin](https://zipkin.io/) service.
+These instructions expect you have [docker-compose](https://docs.docker.com/compose/) installed.
 
-### Run collector
+Bring up the `zipkin-collector` service and example `zipkin-client` service to send an example trace:
+```sh
+docker-compose up --detach zipkin-collector zipkin-client
+```
 
+The `zipkin-client` service sends just one trace and exits. Retrieve the `traceId` generated by the `zipkin-client` service; should be the last line in the logs:
 ```sh
-docker run -d -p 9411:9411 openzipkin/zipkin
+docker-compose logs --tail=1 zipkin-client
 ```
 
-### Run client
+With the `traceId` you can view the trace from the `zipkin-collector` service UI hosted on port `9411`, e.g. with `traceId` of `f5695ba3b2ed00ea583fa4fa0badbeef`:
+http://localhost:9411/zipkin/traces/f5695ba3b2ed00ea583fa4fa0badbeef
 
+Shut down the services when you are finished with the example:
 ```sh
-go build .
-./zipkin
+docker-compose down
 ```