Doc update - adding Enrich e.g. in AspNet Readme

src/OpenTelemetry.Instrumentation.AspNet/README.md

@@ -102,6 +102,41 @@ instrumentation. OpenTelemetry has a concept of
 [Sampler](https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/sdk.md#sampling),
 and the `Filter` option does the filtering *before* the Sampler is invoked.
 
+### Enrich
+
+This option allows one to enrich the activity with additional information
+from the raw `HttpRequest`, `HttpResponse` objects. The `Enrich` action is
+called only when `activity.IsAllDataRequested` is `true`. It contains the
+activity itself (which can be enriched), the name of the event, and the
+actual raw object.
+For event name "OnStartActivity", the actual object will be `HttpRequest`.
+For event name "OnStopActivity", the actual object will be `HttpResponse`
+
+The following code snippet shows how to add additional tags using `Enrich`.
+
+```csharp
+this.tracerProvider = Sdk.CreateTracerProviderBuilder()
+    .AddAspNetInstrumentation(opt => opt.Enrich
+        = (activity, eventName, rawObject) =>
+    {
+        if (eventName.Equals("OnStartActivity"))
+        {
+            if (rawObject is HttpRequest httpRequest)
+            {
+                activity.SetTag("physicalPath", httpRequest.PhysicalPath);
+            }
+        }
+        else if (eventName.Equals("OnStopActivity"))
+        {
+            if (rawObject is HttpResponse httpResponse)
+            {
+                activity.SetTag("responseType", httpResponse.ContentType);
+            }
+        }
+    })
+    .Build();
+```
+
 ### Special topic - Enriching automatically collected telemetry
 
 ASP.NET instrumentation stores the `HttpRequest`, `HttpResponse` objects in the