Update listener docs (#398)

* add listener docs

* removed TODO and added more listener info

* removing invalid route from appupdatelistener

docs/operating/extending.md

@@ -6,13 +6,69 @@ IronFunctions is extensible so you can add custom functionality and extend the p
 
 Listeners are the main way to extend IronFunctions. 
 
-To add listeners, copy `main.go` into your own repo and add your own listener implementations. When ready, 
-compile your main package to create your extended version of IronFunctions.
+You can easily use listeners basically creating a struct with [valid methods](#Listener Requirements) and adding it to the `IronFunctions API`.
 
-### AppListener
+Example:
 
-Implement `ifaces/AppListener` interface, then add it using:
-
-```go
-server.AddAppListener(myAppListener)
 ```
+package main
+
+import (
+    "context"
+
+    "github.com/iron-io/functions/api/server"
+    "github.com/iron-io/functions/api/models"
+)
+
+type myCustomListener struct{}
+
+func (c *myCustomListener) BeforeAppCreate(ctx context.Context, app *models.App) error { return nil }
+func (c *myCustomListener) AfterAppCreate(ctx context.Context, app *models.App) error { return nil }
+
+func (c *myCustomListener) BeforeAppUpdate(ctx context.Context, app *models.App) error { return nil }
+func (c *myCustomListener) AfterAppUpdate(ctx context.Context, app *models.App) error { return nil }
+
+func (c *myCustomListener) BeforeAppDelete(ctx context.Context, appName string) error { return nil }
+func (c *myCustomListener) BeforeAppDelete(ctx context.Context, appName string) error { return nil }
+
+function main () {
+    srv := server.New(/* Here all required parameters to initialize the server */)
+
+    srv.AddAppCreateListener(myCustomListener)
+    srv.AddAppUpdateListener(myCustomListener)
+    srv.AddAppDeleteListener(myCustomListener)
+
+    srv.Run()
+}
+```
+
+#### Listener Requirements
+
+To be a valid listener your struct should respect interfaces combined or alone found in the file [listeners.go](/iron-io/functions/blob/master/api/ifaces/listeners.go)
+
+These are all available listeners:
+
+##### AppCreateListener
+
+_Triggers before and after every app creation that happens in the API_ 
+
+Triggered on requests to the following routes:
+
+- POST /v1/apps
+- POST /v1/apps/:app/routes
+
+##### AppUpdateListener
+
+_Triggers before and after every app updates that happens in the API_
+
+Triggered during requests to the following routes:
+
+- PUT /v1/apps
+
+##### AppDeleteListener
+
+_Triggers before and after every app deletion that happens in the API_
+
+Triggered during requests to the following routes:
+
+- DELETE /v1/apps/app