Add Credits section

Author: donnemartin

README.md

@@ -1724,3 +1724,18 @@ Interested in adding a section or helping complete one in-progress?  [Contribute
 * Consistent hashing
 * Scatter gather
 * [Contribute](#contributing)
+
+## Credits
+
+Credits and sources are provided throughout this repo.
+
+Special thanks to:
+
+* [Hired in tech](http://www.hiredintech.com/system-design/the-system-design-process/)
+* [Cracking the coding interview](https://www.amazon.com/dp/0984782850/)
+* [checkcheckzz/system-design-interview](https://github.com/checkcheckzz/system-design-interview)
+* [shashank88/system_design](https://github.com/shashank88/system_design)
+* [mmcgrana/services-engineering](https://github.com/mmcgrana/services-engineering)
+* [System design cheat sheet](https://gist.github.com/vasanthk/485d1c25737e8e72759f)
+* [A distributed systems reading list](http://dancres.github.io/Pages/)
+* [Cracking the system design interview](http://www.puncsky.com/blog/2016/02/14/crack-the-system-design-interview/)