Update README.md with task 7-bonus

SwitchEngines/README.md

@@ -599,6 +599,30 @@ lcl$ ssh -i keys/tf-cloud-init -o IdentitiesOnly=yes $(terraform output -raw ssh
 ...
 ```
 
+
+### Task #7-bonus: a cluster of 2 ###
+
+**Goal:** use advanced HCL features to handle resource arrays.
+
+Now you can provision an instance and have SSH access to it. Let's write a plan variant to provision a cluster of 2 instances.
+
+You shall:
+
+1. Declare a variable or [_local_](https://developer.hashicorp.com/terraform/language/values/locals) `insance_count`.
+1. Extend your "app_server" resource with 
+  - a [`count` argument](https://developer.hashicorp.com/terraform/language/meta-arguments/count) using `insance_count`,
+  - a `name` argument showing the instance count index.
+1. Extend the networking-related resources with the count argument plus the index where necessary.
+1. Modify your `outputs` to show arrays instead of scalar values. 
+
+Once done and your cluster provisioned, the following command should yield a single element of an output array:
+```shell
+$ terraform output --json instance_public_ip  | jq -r '.[0]'
+```
+
+The same login should work for both the instances!
+
+
 ### Task #8: further practice with Cloud-init  ###
 
 **Goal:** get more acquainted with Cloud-init: change the instance's default