add readme section about custom messages

extension/opampextension/README.md

@@ -43,6 +43,30 @@ extensions:
  endpoint: wss:https://127.0.0.1:4320/v1/opamp
 ```
 
+## Custom Messages
+
+Other components may use a configured OpAMP extension to send and receive custom messages to and from an OpAMP server. Components may use the provided `components.Host` from the Start method in order to get a handle to the registry:
+
+```go
+func Start(_ context.Context, host component.Host) error {
+ ext, ok := host.GetExtensions()[opampExtensionID]
+ if !ok {
+ return fmt.Errorf("opamp extension %q does not exist", opampExtensionID)
+ }
+
+ registry, ok := ext.(opampextension.CustomCapabilityRegistry)
+ if !ok {
+ return fmt.Errorf("extension %q is not an custom message registry", opampExtensionID)
+ }
+
+ // You can now use registry.Register to register a custom capability
+
+ return nil
+}
+```
+
+See the [custom_messages.go](./custom_messages.go) for more information on the custom message API.
+
 ## Status
 
 This OpenTelemetry OpAMP agent extension is intended to support the [OpAMP