doc: add internal modules notice

Explicitly note that these modules should not be used outside of core.

PR-URL: #2523
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>

lib/internal/readme.md

@@ -0,0 +1,4 @@
+# Internal Modules
+
+The modules in `lib/internal` are intended for internal use in Node.js core only, and are not accessible with `require()` from user modules.
+These are subject to change at **any** time. Reliance on these modules outside of core is **not supported** in any way.