Add 20. Valid Parentheses solution to readme

Author: Rion5

readme.md

@@ -6,6 +6,7 @@
 |**3**| **[Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters/)** | **[C#](https://github.com/Rion5/LeetCodeSolutions/blob/master/LeetCode/LongestSubstringWithoutRepeatingCharacters.cs)** | **Medium** | **8%** |
 |**7**| **[Reverse Integer](https://leetcode.com/problems/reverse-integer/)** | **[C#](https://github.com/Rion5/LeetCodeSolutions/blob/master/LeetCode/ReverseInteger.cs)** | **Easy** | **98%** |
 |**9**| **[Palindrome Number](https://leetcode.com/problems/palindrome-number/)** | **[C#](https://github.com/Rion5/LeetCodeSolutions/blob/master/LeetCode/PalindromeNumber.cs)** | **Easy** | **22%** |
+|**20**| **[Valid Parentheses](https://leetcode.com/problems/valid-parentheses/)** | **[C#](https://github.com/Rion5/LeetCodeSolutions/blob/master/LeetCode/ValidParentheses.cs)** | **Easy** | **27%** |
 |**26**| **[Remove Duplicates from Sorted Array](https://leetcode.com/problems/remove-duplicates-from-sorted-array/)** | **[C#](https://github.com/Rion5/LeetCodeSolutions/blob/master/LeetCode/RemoveDuplicatesFromSortedArray.cs)** | **Easy** | **49%** |
 |**35**| **[Search Insert Position](https://leetcode.com/problems/search-insert-position/)** | **[C#](https://github.com/Rion5/LeetCodeSolutions/blob/master/LeetCode/SearchInsertPosition.cs)** | **Easy** | **99%**|
 |**58**| **[Length of Last Word](https://leetcode.com/problems/length-of-last-word/)** | **[C#](https://github.com/Rion5/LeetCodeSolutions/blob/master/LeetCode/LengthOfLastWord.cs)** | **Easy** | **59%**|