Add note on outdated versions in README

Author: jhnns

README.md

@@ -43,3 +43,19 @@ Some modules have mocks in the `mock` directory. These are replacements with min
 | util | [defunctzombie/node-util](https://github.com/defunctzombie/node-util) | --- |
 | vm | [substack/vm-browserify](https://github.com/substack/vm-browserify) | --- |
 | zlib | [devongovett/browserify-zlib](https://github.com/devongovett/browserify-zlib) | --- |
+
+## Outdated versions 
+
+### `buffer`
+
+The current `buffer` implementation uses feross/[email protected] because feross/[email protected] relies on [typed arrays](https://github.com/feross/buffer/commit/5daca86b7cd5d2b8ccb167534d47421029f639e9#commitcomment-19698936).
+This will be dropped as soon as IE9 is not a typical browser target anymore.
+
+### `punycode`
+
+The current `punycode` implementation uses bestiejs/punycode.[email protected] because bestiejs/punycode.[email protected] requires modern JS engines that understand `const` and `let`.
+It will be removed someday since it has already been [deprecated from the node API](https://nodejs.org/api/punycode.html).
+
+## License
+
+MIT