Added README.md file for Target Sum

Author: joshua-dierickse-alt

494-target-sum/README.md

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+<h2><a href="https://leetcode.com/problems/target-sum">Target Sum</a></h2> <img src='https://img.shields.io/badge/Difficulty-Medium-orange' alt='Difficulty: Medium' /><hr><p>You are given an integer array <code>nums</code> and an integer <code>target</code>.</p>
+
+<p>You want to build an <strong>expression</strong> out of nums by adding one of the symbols <code>&#39;+&#39;</code> and <code>&#39;-&#39;</code> before each integer in nums and then concatenate all the integers.</p>
+
+<ul>
+	<li>For example, if <code>nums = [2, 1]</code>, you can add a <code>&#39;+&#39;</code> before <code>2</code> and a <code>&#39;-&#39;</code> before <code>1</code> and concatenate them to build the expression <code>&quot;+2-1&quot;</code>.</li>
+</ul>
+
+<p>Return the number of different <strong>expressions</strong> that you can build, which evaluates to <code>target</code>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre>
+<strong>Input:</strong> nums = [1,1,1,1,1], target = 3
+<strong>Output:</strong> 5
+<strong>Explanation:</strong> There are 5 ways to assign symbols to make the sum of nums be target 3.
+-1 + 1 + 1 + 1 + 1 = 3
++1 - 1 + 1 + 1 + 1 = 3
++1 + 1 - 1 + 1 + 1 = 3
++1 + 1 + 1 - 1 + 1 = 3
++1 + 1 + 1 + 1 - 1 = 3
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre>
+<strong>Input:</strong> nums = [1], target = 1
+<strong>Output:</strong> 1
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= nums.length &lt;= 20</code></li>
+	<li><code>0 &lt;= nums[i] &lt;= 1000</code></li>
+	<li><code>0 &lt;= sum(nums[i]) &lt;= 1000</code></li>
+	<li><code>-1000 &lt;= target &lt;= 1000</code></li>
+</ul>