Added README.md file for Minimum Depth of Binary Tree

111-minimum-depth-of-binary-tree/README.md

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+<h2><a href="https://leetcode.com/problems/minimum-depth-of-binary-tree">Minimum Depth of Binary Tree</a></h2> <img src='https://img.shields.io/badge/Difficulty-Easy-brightgreen' alt='Difficulty: Easy' /><hr><p>Given a binary tree, find its minimum depth.</p>
+
+<p>The minimum depth is the number of nodes along the shortest path from the root node down to the nearest leaf node.</p>
+
+<p><strong>Note:</strong>&nbsp;A leaf is a node with no children.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2020/10/12/ex_depth.jpg" style="width: 432px; height: 302px;" />
+<pre>
+<strong>Input:</strong> root = [3,9,20,null,null,15,7]
+<strong>Output:</strong> 2
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> root = [2,null,3,null,4,null,5,null,6]
+<strong>Output:</strong> 5
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li>The number of nodes in the tree is in the range <code>[0, 10<sup>5</sup>]</code>.</li>
+	<li><code>-1000 &lt;= Node.val &lt;= 1000</code></li>
+</ul>