add find subnets example

archive/find_subnets_example.md

@@ -0,0 +1,14 @@
+```python
+def find_subnets(nodes: list[Node]) -> list[tuple[Node, Node]]:
+    subnets = []
+    for i, (a, b) in enumerate(zip(nodes, nodes[1:])):
+        parent_node, dip = merge_two_nodes(a, b)
+        if parent_node == a or parent_node == b:
+            subnets.append((a, b))
+    return subnets
+
+
+def ensure_no_subnets(nodes: list[Node]):
+    if subnets := find_subnets(nodes):
+        raise Cidr4MergerError(f"There are subnets! {subnets=}")
+```