Add README for the HOME_NET use case

network/network-firewall/README.md

@@ -0,0 +1,28 @@
+# Domain list inspection for traffic from outside the Network Firewall VPC
+To use domain name filtering for traffic from outside the VPC where you've deployed Network Firewall, you must manually set the `HOME_NET` variable for the rule group. The most common use case for this is a central firewall VPC with traffic coming from other VPCs through a transit gateway.
+
+Include the `HOME_NET` variable in the dtaeful group definiton as follow:
+
+```
+  # Stateful rules
+  stateful_rule_groups = {
+    # rules_source_list examples
+    stateful-group-1 = {
+      description = "Stateful Inspection for denying access to domains"
+      capacity    = 100
+      rule_variables = {
+        ip_sets = {
+          HOME_NET = ["0.0.0.0/0"]
+        }
+      }
+      rules_source_list = {
+        generated_rules_type = "DENYLIST"
+        target_types         = ["TLS_SNI", "HTTP_HOST"]
+        targets              = [".wikipedia.org", ".bad-domain.com"]
+      }
+    }
+  }
+}
+```
+**Reference**: [Domain list inspection for traffic from outside the deployment VPC](https://docs.aws.amazon.com/network-firewall/latest/developerguide/stateful-rule-groups-domain-names.html#:~:text=see%20Domain%20filtering.-,Domain%20list%20inspection%20for%20traffic%20from%20outside%20the%20deployment%20VPC,-To%20use%20domain)
+