Add the System Design Primer

Resource to learn how to design large-scale systems and to prep for the system design interview. Resolves jwasham#174.

README.md

@@ -1195,7 +1195,8 @@ You'll get more graph practice in Skiena's book (see Books section below) and th
         - simplicity and robustness
         - tradeoffs
         - performance analysis and optimization
-- [ ] **START HERE**: [System Design from HiredInTech](http://www.hiredintech.com/system-design/)
+- [ ] **START HERE**: [The System Design Primer](https://github.com/donnemartin/system-design-primer)
+- [ ] [System Design from HiredInTech](http://www.hiredintech.com/system-design/)
 - [ ] [How Do I Prepare To Answer Design Questions In A Technical Inverview?](https://www.quora.com/How-do-I-prepare-to-answer-design-questions-in-a-technical-interview?redirected_qid=1500023)
 - [ ] [8 Things You Need to Know Before a System Design Interview](http://blog.gainlo.co/index.php/2015/10/22/8-things-you-need-to-know-before-system-design-interviews/)
 - [ ] [Algorithm design](http://www.hiredintech.com/algorithm-design/)