Add some reference links

TheAlgorithms · dhruvmanila · Nov 21, 2020 · Oct 5, 2020 · Oct 5, 2020 · Oct 6, 2020
commit bee3abf185a9ea7fcab8727c9c88bf0a359af9ba
diff --git a/project_euler/problem_188/sol1.py b/project_euler/problem_188/sol1.py
@@ -12,6 +12,12 @@
 3↑↑4 is roughly 103.6383346400240996*10^12.
 
 Find the last 8 digits of 1777↑↑1855.
+
+References:
+    - The original Project Euler project page:
+      https://projecteuler.net/problem=188
+    - Wikipedia article about Hyperexponentiation, aka. Tetration
+      https://en.wikipedia.org/wiki/Tetration
 """