feat(challenge): first non-repeating character

challenges/first-non-repeating-character/README.md

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+# first-non-repeating-character
+
+Created by [@Divlo](https://github.com/Divlo) on 15 November 2020.
+
+## Instructions :
+
+Write a function that takes a string input, and returns the first character that is not repeated anywhere in the string.
+
+For example, if given the input `'stress'`, the function should return `'t'`, since the letter _t_ only occurs once in the string, and occurs first in the string.
+
+If a string contains all repeating characters, it should return an empty string (`""`).
+
+## Source :
+
+[First non-repeating character - Codewars](https://www.codewars.com/kata/52bc74d4ac05d0945d00054e/)
+
+## Examples :
+
+See the `input-output.json` file for examples of input/output.