Added README.md file for Largest Rectangle in Histogram

Author: joshua-dierickse-alt

84-largest-rectangle-in-histogram/README.md

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+<h2><a href="https://leetcode.com/problems/largest-rectangle-in-histogram">Largest Rectangle in Histogram</a></h2> <img src='https://img.shields.io/badge/Difficulty-Hard-red' alt='Difficulty: Hard' /><hr><p>Given an array of integers <code>heights</code> representing the histogram&#39;s bar height where the width of each bar is <code>1</code>, return <em>the area of the largest rectangle in the histogram</em>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2021/01/04/histogram.jpg" style="width: 522px; height: 242px;" />
+<pre>
+<strong>Input:</strong> heights = [2,1,5,6,2,3]
+<strong>Output:</strong> 10
+<strong>Explanation:</strong> The above is a histogram where width of each bar is 1.
+The largest rectangle is shown in the red area, which has an area = 10 units.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2021/01/04/histogram-1.jpg" style="width: 202px; height: 362px;" />
+<pre>
+<strong>Input:</strong> heights = [2,4]
+<strong>Output:</strong> 4
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= heights.length &lt;= 10<sup>5</sup></code></li>
+	<li><code>0 &lt;= heights[i] &lt;= 10<sup>4</sup></code></li>
+</ul>