reverse-string: Add new exercise

exercises/reverse-string/canonical-data.json

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+{
+  "exercise": "reverse-string",
+  "version": "1.0.0",      
+  "comments": [
+        "If property based testing tools are available, a good property to test is reversing a string twice: reverse(reverse(string)) == string"
+      ],
+  "cases": [
+    {
+      "description": "empty string",
+      "property": "reverse",
+      "input": "",
+      "expected": ""
+    },
+    {
+      "description": "a word",
+      "property": "reverse",
+      "input": "robot",
+      "expected": "tobor"
+    },
+    {
+      "description": "a capitalized word",
+      "property": "reverse",
+      "input": "Ramen",
+      "expected": "nemaR"
+    },
+    {
+      "description": "a sentence with punctuation",
+      "property": "reverse",  
+      "input": "I'm hungry!",
+      "expected": "!yrgnuh m'I"
+    },
+    {
+      "description": "a palindrome",
+      "property": "reverse",
+      "input": "racecar",
+      "expected": "racecar"
+    }
+  ]
+}

exercises/reverse-string/description.md

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+Reverse a string
+
+For example:
+input: "cool"
+output: "looc"

exercises/reverse-string/metadata.yml

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+---
+blurb: "Reverse a string"
+title: "reverse-string"
+source: "Introductory challenge to reverse an input string"
+source_url: "https://medium.freecodecamp.org/how-to-reverse-a-string-in-javascript-in-3-different-ways-75e4763c68cb"