feat(challenge): add caesar-cipher

challenges/caesar-cipher/README.md

@@ -0,0 +1,29 @@
+# caesar-cipher
+
+Created by [@Divlo](https://github.com/Divlo) on 21 October 2020.
+
+## Instructions :
+
+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.
+
+### Example of the alphabet with a rotation by 3 :
+
+```
+Original alphabet   : ABCDEFGHIJKLMNOPQRSTUVWXYZ
+Alphabet rotated +3 : DEFGHIJKLMNOPQRSTUVWXYZABC
+```
+
+Complete the solution function. It should return the encrypted string.
+
+The function has the following parameter(s):
+
+- `string`: a string in cleartext (everything uppercase)
+- `shift`: an integer, the alphabet rotation factor
+
+## Source :
+
+- [Wikipedia - Caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher)
+
+## Examples :
+
+See the `input-output.json` file for examples of input/output.