chore: add known-issues.md file

docs/known-issues.md

+# Known issues
+
+## Manual Deletion Required for Static IP Address
+
+This module has `prevent_deletion` attribute enabled for `google_compute_address.static_ip`
+resource. This attribute is used to prevent accidental deletion of important resources.
+It was done on purpose, as it is not expected that static IPs to be regularly removed
+and recreated, as it's likely that they will be used within firewall, DNS records and so on.
+Terraform will display error trying to destroy the resource:
+
+```sh
+│ Error: Instance cannot be destroyed
+│
+│   on .terraform/modules/webapp/static_egress_ip.tf line 40:
+│   40: resource "google_compute_address" "static_ip" {
+│
+│ Resource module.webapp.google_compute_address.static_ip[0] has lifecycle.prevent_destroy set, but the plan calls for this resource to be destroyed. To avoid
+│ this error and continue with the plan, either disable lifecycle.prevent_destroy or reduce the scope of the plan using the -target flag.
+```
+
+However, sometimes it needs to be deleted. If a resource is created with `prevent_deletion`,
+Terraform will block any attempt to delete this resource through Terraform commands.
+As a result, if you need to remove such a resource from your Terraform configuration,
+you must manually delete the resource outside of Terraform before running `terraform apply` again.
+
+The steps below shows how it can be achieved.
+
+## Steps to Resolve
+
+First, delete the resources that use the IP address. In common case it is Cloud Router with the
+default name `webapp-ip-router`. If not sure, check the list of routers using `gcloud` command:
+
+```sh
+gcloud compute routers list --project $PROJECT_ID
+
+NAME              REGION        NETWORK
+webapp-ip-router  europe-west2  default
+```
+
+Delete the Cloud Router:
+
+```sh
+gcloud compute routers delete webapp-ip-router --project $PROJECT_ID --region $REGION
+```
+
+Now the IP adress can be deleted:
+
+```sh
+gcloud compute addresses delete webapp-static-ip --region $REGION --project $PROJECT_ID
+```
+
+That's it. Now Terraform commands will work without errors.