Update readme with an example for Jetty Http Client instrumentation. (#1671)

Author: rghetia

contrib/http_jetty_client/README.md

@@ -31,6 +31,55 @@ compile 'io.opencensus:opencensus-api:0.19.0'
 compile 'io.opencensus:opencensus-contrib-http-jetty-client:0.19.0'
 ```
 
+## Instrumenting Jetty Http Client
+
+Users can instrument Jetty Http Client by simply creating HttpClient using wrapper OcJettyHttpClient.
+
+Below is an example of how it is instrumented.
+
+```java
+
+import io.opencensus.contrib.http.jetty.client.OcJettyHttpClient;
+import io.opencensus.contrib.http.util.HttpViews;
+import io.opencensus.exporter.stats.prometheus.PrometheusStatsCollector;
+import io.opencensus.exporter.trace.logging.LoggingTraceExporter;
+
+
+class Application {
+  private static void initStatsExporter() throws IOException {
+    
+    HttpViews.registerAllClientViews();
+    
+    // Register stats exporter for desired backend.
+    PrometheusStatsCollector.createAndRegister();
+    HTTPServer prometheusServer = new HTTPServer(9091, true);
+  }
+
+  public static void main(String[] args) throws Exception {
+    BasicConfigurator.configure();
+
+    // Register trace exporter for desired backend. 
+    LoggingTraceExporter.register();
+
+    // Initialize Stats Exporter
+    initStatsExporter();
+
+    OcJettyHttpClient httpClient = new OcJettyHttpClient();
+
+    httpClient.start();
+
+    HttpRequest request =
+        (HttpRequest) httpClient.newRequest("http://example.com/").method(HttpMethod.GET);
+    request.send();
+
+    HttpRequest postRequest =
+        (HttpRequest) httpClient.newRequest("http://example.com/").method(HttpMethod.POST);
+    postRequest.content(new StringContentProvider("{\"hello\": \"world\"}"), "application/json");
+    postRequest.send();
+  }
+}
+```
+
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