[Hacker Rank]: Project Euler #1: Multiples of 3 and 5. Documentation added.

Author: Gonzalo Diaz

algorithm-exercises-csharp/src/hackerrank/projecteuler/Euler001.cs

@@ -1,3 +1,5 @@
+// @link Problem definition [[docs/hackerrank/projecteuler/euler001.md]]
+
 namespace algorithm_exercises_csharp.hackerrank.prohecteuler;
 
 using System.Diagnostics.CodeAnalysis;

docs/.gitkeep

@@ -0,0 +1,49 @@
+# [Multiples of 3 and 5](https://www.hackerrank.com/contests/projecteuler/challenges/euler001)
+
+- Difficulty: #easy
+- Category: #ProjectEuler+
+
+If we list all the natural numbers below $ 10 $ that are multiples of
+ $ 3 $ or $ 5 $, we get $ 3, 5, 6 $ and $ 9 $.
+ The sum of these multiples
+ is $ 23 $.
+
+Find the sum of all the multiples of $ 3 $ or $ 5 $ below $ N $.
+
+## Input Format
+
+First line contains $ T $ that denotes the number of test cases.
+This is followed by $ T $ lines, each containing an integer, $ N $.
+
+## Constraints
+
+- 1 $ \leq T \leq 10^5 $
+- 1 $ \leq N \leq 10^9 $
+
+## Output Format
+
+For each test case, print an integer that denotes the sum of all the multiples
+of $ 3 $ or $ 5 $ below $ N $.
+
+## Sample Input 0
+
+```text
+2
+10
+100
+```
+
+## Sample Output 0
+
+```text
+23
+2318
+```
+
+## Explanation 0
+
+For $ N = 10 $, if we list all the natural numbers below $ 10 $ that are
+ multiples of $ 3 $ or $ 5 $, we get $ 3, 5, 6 $ and $ 9 $.
+  The sum of these multiples is $ 23 $.
+
+Similarly for $ N = 100 $, we get $ 2318 $.