added variable description

README.md

@@ -251,6 +251,7 @@ Available targets:
 
 | Name | Description | Type | Default | Required |
 |------|-------------|------|---------|:--------:|
+| additional\_security\_group\_ids | Additional list of security groups that will be attached to the node group | `list(string)` | `[]` | no |
 | additional\_tag\_map | Additional tags for appending to tags\_as\_list\_of\_maps. Not added to `tags`. | `map(string)` | `{}` | no |
 | after\_cluster\_joining\_userdata | Additional `bash` commands to execute on each worker node after joining the EKS cluster (after executing the `bootstrap.sh` script). For more info, see https://kubedex.com/90-days-of-aws-eks-in-production | `string` | `""` | no |
 | ami\_image\_id | AMI to use. Ignored of `launch_template_id` is supplied. | `string` | `null` | no |