feat(challenges): add caesar-cipher

challenges/caesar-cipher/README.md

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+# caesar-cipher
+
+Created by [@Divlo](https://github.com/Divlo) on 25 June 2021.
+
+## 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 shift of +3 (shift to the right)
+
+```text
+Alphabet original   : ABCDEFGHIJKLMNOPQRSTUVWXYZ
+Alphabet rotated +3 : DEFGHIJKLMNOPQRSTUVWXYZABC
+```
+
+Create a function that will return the sentence after shifting the alphabet.
+
+- If it is a **positive** number then we shift the alphabet to the **right**
+- If it is a **negative** number then we shift the alphabet to the **left**
+
+### Example of Inputs
+
+```py
+'ANTHONY' # a character string (all capital letters)
+'-2' # an integer, the shift in the alphabet
+```
+
+## Source
+
+- [Wikipedia - Caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher)
+
+## Examples
+
+See the `test` folder for examples of input/output.