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 <[email protected]>
Reviewed-By: Colin Ihrig <[email protected]>
Reviewed-By: Sakthipriyan Vairamani <[email protected]>
Reviewed-By: Michaël Zasso <[email protected]>

Author: Fishrock123

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.