docs(app-sdk): Add background publisher API detail

docs_src/microservices/application/AdvancedTopics.md

@@ -424,6 +424,61 @@ If in secure mode, the secrets are retrieved from the secret store based on the
 
 If running in insecure mode, the secrets are retrieved from the *Writable.InsecureSecrets* configuration.
 
+### Background Publishing
+
+Application Services using the MessageBus trigger can request a background publisher using the AddBackgroundPublisher API in the SDK.  This method takes an int representing the background channel's capacity as the only parameter and returns a reference to a BackgroundPublisher.  This reference can then be used by background processes to publish to the configured MessageBus output.  Example usage is as follows:
+
+```go
+
+func runJob (pub appsdk.BackgroundPublisher, done chan struct{}){
+	ticker := time.NewTicker(1 * time.Minute)
+	go func() {
+		for {
+			select {
+			case <-ticker.C:
+				msg := myDataService.GetMessage()
+				payload, err := json.Marshal(message)
+
+				if err != nil {
+					continue
+				}
+				pub.Publish(payload, uuid.New().String(), clients.ContentTypeJSON)
+			case <-j.done:
+				ticker.Stop()
+				return
+			}
+		}
+	}()
+}
+
+func main() {
+	edgexSdk := &appsdk.AppFunctionsSDK{ServiceKey: serviceKey}
+	if err := edgexSdk.Initialize(); err != nil {
+		edgexSdk.LoggingClient.Error(fmt.Sprintf("SDK initialization failed: %v", err))
+		os.Exit(-1)
+	}
+
+	//initialize background publisher with a channel capacity of 10
+	pub := sdk.AddBackgroundPublisher(10)
+
+	done := make(chan struct{})
+	defer close(done)
+
+	//pass publisher to your background job
+	runJob(pub, done)
+
+	edgexSdk.SetFunctionsPipeline(
+		All,
+		My,
+		Functions,
+	)
+
+	edgexSdk.MakeItRun()
+
+	os.Exit(0)
+}
+```
+
 ### Registry Client
 
 **After initialization**, the configured registry client used by the SDK can be retrieved from the sdk instance at .RegistryClient.  It is important to note that sdk.RegistryClient may be nil - either if the SDK is not yet initialized, or if the registry option (-r/--registry) is not specified on start.  Once retrieved the client can be used to look up host information for other services, or perform other operations supported by the registry.Client type in [go-mod-registry](https://github.com/edgexfoundry/go-mod-registry).  For example, to retrieve the URL for a given service:
@@ -445,4 +500,4 @@ func(sdk *appsdk.AppFunctionsSDK, serviceKey string) (string, error) {
 ```
 !!! note Known Service Keys
     Service keys for known EdgeX services can be found under clients in [go-mod-core-contracts](https://github.com/edgexfoundry/go-mod-core-contracts/blob/master/clients/constants.go#L58-L72)
-
+