Refactor a Caesar Cipher algorithm.

src/algorithms/cryptography/caesar-cipher/README.md

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+# Caesar Cipher Algorithm
+
+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.
+
+![Caesar Cipher Algorithm](https://upload.wikimedia.org/wikipedia/commons/4/4a/Caesar_cipher_left_shift_of_3.svg)
+
+## Example
+
+The transformation can be represented by aligning two alphabets; the cipher alphabet is the plain alphabet rotated left or right by some number of positions. For instance, here is a Caesar cipher using a left rotation of three places, equivalent to a right shift of 23 (the shift parameter is used as the key):
+
+```text
+Plain:    ABCDEFGHIJKLMNOPQRSTUVWXYZ
+Cipher:   XYZABCDEFGHIJKLMNOPQRSTUVW
+```
+
+When encrypting, a person looks up each letter of the message in the "plain" line and writes down the corresponding letter in the "cipher" line.
+
+```text
+Plaintext:  THE QUICK BROWN FOX JUMPS OVER THE LAZY DOG
+Ciphertext: QEB NRFZH YOLTK CLU GRJMP LSBO QEB IXWV ALD
+```
+
+## Complexity
+
+- Time: `O(|n|)`
+- Space: `O(|n|)`
+
+## References
+
+- [Caesar cipher on Wikipedia](https://en.wikipedia.org/wiki/Caesar_cipher)