docs: add migration guide for webpack-dev-server@6.0.0

Author: bjohansebas

migration-v6.md

@@ -0,0 +1,19 @@
+# Migration guide
+
+This document serves as a migration guide for `webpack-dev-server@6.0.0`.
+
+## Deprecations
+
+- The static methods `internalIP` and `internalIPSync` were removed. Use `findIp` instead.
+
+  v4:
+
+  ```js
+  const ip = Server.internalIP("v4");
+  ```
+
+  v5:
+
+  ```js
+  const ip = Server.findIp("v4", true);
+  ```