Add deployment slot example

The existing documentation does not include an example that shows how to create a deployment slot for a web app. After much frustration, I finally had to break out fiddler and look at the actual request that was being made in order to figure out how to create one. Hopefully the addition of this example will help someone like me who is struggling to easily solve this problem.

+label: docsite_pr

lib/ansible/modules/cloud/azure/azure_rm_webapp.py

@@ -253,6 +253,16 @@ EXAMPLES = '''
             version: 6.6
           - name: "php"
             version: "7.0"
+    
+    - name: Create a stage deployment slot for an existing web app
+      azure_rm_webapp:
+        resource_group: myresourcegroup
+        name: mywebapp/slots/stage
+        plan:
+          resource_group: myresourcegroup
+          name: myappplan
+        app_settings:
+          testkey:testvalue      
 
     - name: Create a linux web app with java framework
       azure_rm_webapp: