Docs: Note custom mkdirp implementation

Author: phated

LICENSE

@@ -1,6 +1,6 @@
 The MIT License (MIT)
 
-Copyright (c) 2017 Blaine Bublitz <blaine.bublitz@gmail.com>, Eric Schoffstall <yo@contra.io> and other contributors
+Copyright (c) 2017 Blaine Bublitz <blaine.bublitz@gmail.com>, Eric Schoffstall <yo@contra.io> and other contributors (Originally based on code from node-mkdirp - MIT/X11 license - Copyright 2010 James Halliday)
 
 Permission is hereby granted, free of charge, to any person obtaining a copy
 of this software and associated documentation files (the "Software"), to deal

README.md

@@ -46,6 +46,8 @@ The same as the top-level API but for object streams. See the example to see the
 
 MIT
 
+Contains a custom implementation of `mkdirp` originally based on https://github.com/substack/node-mkdirp (Licensed MIT/X11 - Copyright 2010 James Halliday) with heavy modification to better support custom modes.
+
 [downloads-image]: http://img.shields.io/npm/dm/fs-mkdirp-stream.svg
 [npm-url]: https://npmjs.com/package/fs-mkdirp-stream
 [npm-image]: http://img.shields.io/npm/v/fs-mkdirp-stream.svg