add project euler

website with lots of problems of varying difficulty that take a numerical answer instead of being evaluated by an online judge.

Author: eshrh

README.md

@@ -159,3 +159,4 @@ But knowing the stuff will help you become better! :muscle:*
 - :link: [Spoj](https://spoj.com/)
 - :link: [HackerEarth](https://hackerearth.com/)
 - :link: [TopCoder](https://www.topcoder.com/)
+- :link: [Project Euler](https://projecteuler.net/)