Resolve #281: Add design stock exchange links (#441)

* Additional question, build an exchange

fixes donnemartin/system-design-primer#281

* Update README.md

Co-authored-by: Donne Martin <donne.martin@gmail.com>

README.md

@@ -1673,6 +1673,7 @@ Handy metrics based on numbers above:
 | Design an online multiplayer card game | [indieflashblog.com](http://www.indieflashblog.com/how-to-create-an-asynchronous-multiplayer-game.html)<br/>[buildnewgames.com](http://buildnewgames.com/real-time-multiplayer/) |
 | Design a garbage collection system | [stuffwithstuff.com](http://journal.stuffwithstuff.com/2013/12/08/babys-first-garbage-collector/)<br/>[washington.edu](http://courses.cs.washington.edu/courses/csep521/07wi/prj/rick.pdf) |
 | Design an API rate limiter | [https://stripe.com/blog/](https://stripe.com/blog/rate-limiters) |
+| Design a Stock Exchange (like NASDAQ or Binance) | [Jane Street](https://youtu.be/b1e4t2k2KJY)<br/>[Golang Implementation](https://around25.com/blog/building-a-trading-engine-for-a-crypto-exchange/)<br/>[Go Implemenation](http://bhomnick.net/building-a-simple-limit-order-in-go/) |
 | Add a system design question | [Contribute](#contributing) |
 
 ### Real world architectures