Add the Project Euler to the list of Challenge sites.

README.md

@@ -560,6 +560,7 @@ Challenge sites:
 - [TopCoder](https://www.topcoder.com/)
 - [Geeks for Geeks](https://practice.geeksforgeeks.org/explore/?page=1)
 - [InterviewBit](https://www.interviewbit.com/)
+- [Project Euler](https://projecteuler.net/)
 
 ## The Study Plan