Added README.md file for Constrained Subsequence Sum

Co-authored-by: Joshua Dierickse <joshua.dierickse@gmail.com>

Author: 2 people

1286-constrained-subsequence-sum/README.md

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+<h2><a href="https://leetcode.com/problems/constrained-subsequence-sum">Constrained Subsequence Sum</a></h2> <img src='https://img.shields.io/badge/Difficulty-Hard-red' alt='Difficulty: Hard' /><hr><p>Given an integer array <code>nums</code> and an integer <code>k</code>, return the maximum sum of a <strong>non-empty</strong> subsequence of that array such that for every two <strong>consecutive</strong> integers in the subsequence, <code>nums[i]</code> and <code>nums[j]</code>, where <code>i &lt; j</code>, the condition <code>j - i &lt;= k</code> is satisfied.</p>
+
+<p>A <em>subsequence</em> of an array is obtained by deleting some number of elements (can be zero) from the array, leaving the remaining elements in their original order.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> nums = [10,2,-10,5,20], k = 2
+<strong>Output:</strong> 37
+<b>Explanation:</b> The subsequence is [10, 2, 5, 20].
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> nums = [-1,-2,-3], k = 1
+<strong>Output:</strong> -1
+<b>Explanation:</b> The subsequence must be non-empty, so we choose the largest number.
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+
+<pre>
+<strong>Input:</strong> nums = [10,-2,-10,-5,20], k = 2
+<strong>Output:</strong> 23
+<b>Explanation:</b> The subsequence is [10, -2, -5, 20].
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= k &lt;= nums.length &lt;= 10<sup>5</sup></code></li>
+	<li><code>-10<sup>4</sup> &lt;= nums[i] &lt;= 10<sup>4</sup></code></li>
+</ul>