docs(crossword): Add subject for new piscine-js raid 1

subjects/crossword/README.md

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+## crossword
+
+### Instructions
+
+Create the function `crosswordSolver` that is able to solve an empty crossword puzzle 
+
+The function takes two arguments:
+1. an empty puzzle, passed as a string and
+2. a list of words to fill in the puzzle (no double words allowed)
+
+The function must return the puzzle filled with the input words.
+
+The empty puzzle will be a string with the following rules:
+- each character will be either a number, a `.` or a `\n`;
+- a number represent the number of words starting from the specific position and a `.` represent a space that does not need to be filled. 
+
+If the puzzle or list of words provided as inputs does not guarantee a unique solution, or any other conditions stated above are not met, the function must return `'Error'`. 
+
+### Examples
+
+```js
+console.log(crosswordSolver('2001\n0..0\n1000\n0..0', ['casa', 'alan', 'ciao', 'anta']))
+// output: 'casa\ni..l\nanta\no..n'
+```