Added README.md file for Find the Index of the First Occurrence in a …

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+<h2><a href="https://leetcode.com/problems/find-the-index-of-the-first-occurrence-in-a-string">Find the Index of the First Occurrence in a String</a></h2> <img src='https://img.shields.io/badge/Difficulty-Easy-brightgreen' alt='Difficulty: Easy' /><hr><p>Given two strings <code>needle</code> and <code>haystack</code>, return the index of the first occurrence of <code>needle</code> in <code>haystack</code>, or <code>-1</code> if <code>needle</code> is not part of <code>haystack</code>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> haystack = &quot;sadbutsad&quot;, needle = &quot;sad&quot;
+<strong>Output:</strong> 0
+<strong>Explanation:</strong> &quot;sad&quot; occurs at index 0 and 6.
+The first occurrence is at index 0, so we return 0.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> haystack = &quot;leetcode&quot;, needle = &quot;leeto&quot;
+<strong>Output:</strong> -1
+<strong>Explanation:</strong> &quot;leeto&quot; did not occur in &quot;leetcode&quot;, so we return -1.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= haystack.length, needle.length &lt;= 10<sup>4</sup></code></li>
+	<li><code>haystack</code> and <code>needle</code> consist of only lowercase English characters.</li>
+</ul>