Added README.md file for Matrix Diagonal Sum

Author: achuatchaya

1677-matrix-diagonal-sum/README.md

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+<h2><a href="https://leetcode.com/problems/matrix-diagonal-sum">Matrix Diagonal Sum</a></h2> <img src='https://img.shields.io/badge/Difficulty-Easy-brightgreen' alt='Difficulty: Easy' /><hr><p>Given a&nbsp;square&nbsp;matrix&nbsp;<code>mat</code>, return the sum of the matrix diagonals.</p>
+
+<p>Only include the sum of all the elements on the primary diagonal and all the elements on the secondary diagonal that are not part of the primary diagonal.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+<img alt="" src="https://assets.leetcode.com/uploads/2020/08/14/sample_1911.png" style="width: 336px; height: 174px;" />
+<pre>
+<strong>Input:</strong> mat = [[<strong>1</strong>,2,<strong>3</strong>],
+&nbsp;             [4,<strong>5</strong>,6],
+&nbsp;             [<strong>7</strong>,8,<strong>9</strong>]]
+<strong>Output:</strong> 25
+<strong>Explanation: </strong>Diagonals sum: 1 + 5 + 9 + 3 + 7 = 25
+Notice that element mat[1][1] = 5 is counted only once.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> mat = [[<strong>1</strong>,1,1,<strong>1</strong>],
+&nbsp;             [1,<strong>1</strong>,<strong>1</strong>,1],
+&nbsp;             [1,<strong>1</strong>,<strong>1</strong>,1],
+&nbsp;             [<strong>1</strong>,1,1,<strong>1</strong>]]
+<strong>Output:</strong> 8
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> mat = [[<strong>5</strong>]]
+<strong>Output:</strong> 5
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>n == mat.length == mat[i].length</code></li>
+	<li><code>1 &lt;= n &lt;= 100</code></li>
+	<li><code>1 &lt;= mat[i][j] &lt;= 100</code></li>
+</ul>