add caesar cipher

caesar/README.md

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+# Exercise XX - caesar cipher
+
+Implement the legendary caesar cipher:
+
+> In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher, Caesar's code or Caesar shift, is one of the simplest and most widely known encryption techniques. It is a type of substitution cipher in which each letter in the plaintext is replaced by a letter some fixed number of positions down the alphabet. For example, with a left shift of 3, D would be replaced by A, E would become B, and so on. The method is named after Julius Caesar, who used it in his private correspondence.
+
+write a function that takes a string to be encoded and a shift factor and then returns the encoded string:
+
+```javascript
+caesar('A', 1) // simply shifts the letter by 1: returns 'B'
+```
+
+the cipher should retain capitalization:
+```javascript
+caesar('Hey', 5) // returns 'Mjd;
+```
+
+should _not_ shift punctuation:
+```javascript
+caesar('Hello, World!', 5) //returns 'Mjqqt, Btwqi!'
+```
+
+the shift should wrap around the alphabet:
+```javascript
+caesar('Z', 1) // returns 'A'
+```
+
+negative numbers should work as well:
+```javascript
+caesar('Mjqqt, Btwqi!', -5) // returns 'Hello, World!'
+```
+
+