Added README.md file for Number of Common Factors

Author: achuatchaya

2507-number-of-common-factors/README.md

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+<h2><a href="https://leetcode.com/problems/number-of-common-factors">Number of Common Factors</a></h2> <img src='https://img.shields.io/badge/Difficulty-Easy-brightgreen' alt='Difficulty: Easy' /><hr><p>Given two positive integers <code>a</code> and <code>b</code>, return <em>the number of <strong>common</strong> factors of </em><code>a</code><em> and </em><code>b</code>.</p>
+
+<p>An integer <code>x</code> is a <strong>common factor</strong> of <code>a</code> and <code>b</code> if <code>x</code> divides both <code>a</code> and <code>b</code>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> a = 12, b = 6
+<strong>Output:</strong> 4
+<strong>Explanation:</strong> The common factors of 12 and 6 are 1, 2, 3, 6.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> a = 25, b = 30
+<strong>Output:</strong> 2
+<strong>Explanation:</strong> The common factors of 25 and 30 are 1, 5.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= a, b &lt;= 1000</code></li>
+</ul>