Add route for async invocation of functions in the kong guide

With this commit i extend the kong docs to add routing for
async invocation of functions.

Signed-off-by: Martin Dekov (VMware) <mdekov@vmware.com>

guide/kong_integration.md

@@ -41,7 +41,7 @@ $ docker service create --name kong-database \
     -e "POSTGRES_USER=kong" \
     -e "POSTGRES_DB=kong" \
     -e "POSTGRES_PASSWORD=secretpassword" \
-    postgres:9.4
+    postgres:9.5
 ```
 
 Now we will use the Kong image to populate default configuration in the Postgres database:
@@ -103,6 +103,18 @@ $ curl localhost:8000/function/func_echoit -d 'hello world'
 hello world
 ```
 
+Also enable routing for async invocation of functions
+```
+$ kong_admin_curl -X POST \
+    --url http://localhost:8001/apis/ \
+    --data 'name=async-function' \
+    --data 'uris=/async-function' \
+    --data 'upstream_url=http://gateway:8080/async-function'
+
+$ curl localhost:8000/async-function/func_echoit -d 'hello world'
+hello world
+```
+
 ### Require basic authentication
 
 Enable the basic-auth plugin in Kong