Added README.md file for Add Strings

Author: ErAdnanHabib

415-add-strings/README.md

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+<h2><a href="https://leetcode.com/problems/add-strings">Add Strings</a></h2> <img src='https://img.shields.io/badge/Difficulty-Easy-brightgreen' alt='Difficulty: Easy' /><hr><p>Given two non-negative integers, <code>num1</code> and <code>num2</code> represented as string, return <em>the sum of</em> <code>num1</code> <em>and</em> <code>num2</code> <em>as a string</em>.</p>
+
+<p>You must solve the problem without using any built-in library for handling large integers (such as <code>BigInteger</code>). You must also not convert the inputs to integers directly.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> num1 = &quot;11&quot;, num2 = &quot;123&quot;
+<strong>Output:</strong> &quot;134&quot;
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> num1 = &quot;456&quot;, num2 = &quot;77&quot;
+<strong>Output:</strong> &quot;533&quot;
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> num1 = &quot;0&quot;, num2 = &quot;0&quot;
+<strong>Output:</strong> &quot;0&quot;
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+<li><code>1 &lt;= num1.length, num2.length &lt;= 10<sup>4</sup></code></li>
+<li><code>num1</code> and <code>num2</code> consist of only digits.</li>
+<li><code>num1</code> and <code>num2</code> don&#39;t have any leading zeros except for the zero itself.</li>
+</ul>