Update READMEs.

Author: trekhleb

src/algorithms/sets/maximum-subarray/README.md

@@ -6,6 +6,8 @@ which has the largest sum, where,
 
 ![Maximum subarray](https://wikimedia.org/api/rest_v1/media/math/render/svg/e8960f093107b71b21827e726e2bad8b023779b2)
 
+![Maximum subarray](https://www.geeksforgeeks.org/wp-content/uploads/kadane-Algorithm.png)
+
 ## Example
 
 The list usually contains both positive and negative numbers along 
@@ -17,3 +19,4 @@ with the largest sum is `4, −1, 2, 1`, with sum `6`.
 
 - [Wikipedia](https://en.wikipedia.org/wiki/Maximum_subarray_problem)
 - [YouTube](https://www.youtube.com/watch?v=ohHWQf1HDfU&list=PLLXdhg_r2hKA7DPDsunoDZ-Z769jWn4R8)
+- [GeeksForGeeks](https://www.geeksforgeeks.org/largest-sum-contiguous-subarray/)