Update problem file

Author: deniscostadsc

solutions/project-euler/028/problem.md

@@ -0,0 +1,19 @@
+https://projecteuler.net/problem=28
+
+# Number Spiral Diagonals
+
+Starting with the number $1$ and moving to the right in a clockwise direction a
+$5$ by $5$ spiral is formed as follows:
+
+```
+21 22 23 24 25
+20  7  8  9 10
+19  6  1  2 11
+18  5  4  3 12
+17 16 15 14 13
+```
+
+It can be verified that the sum of the numbers on the diagonals is $101$.
+
+What is the sum of the numbers on the diagonals in a $1001$ by $1001$ spiral
+formed in the same way?