feat(minesweeper): new optional exercise for rust piscine

subjects/minesweeper/README.md

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+## minesweeper
+
+Create a function that takes a minesweeper board as an array of strings and return the board solved.
+
+Minesweeper is a very old game where some mines are placed in a board and you should calculate how many mines are touching every free field and write the count in the respective place.
+
+> We will only test your function with empty and valid boards.
+
+### Instructions
+
+### Expected Function
+
+```rust
+pub fn solve_board(minefield: &[&str]) -> Vec<String> {
+}
+```
+
+### Usage
+
+Here is a possible program to test your function,
+
+```rust
+fn main() {
+    println!("{:?}", solve_board(&[]));
+    println!("{:?}", solve_board(&[""]));
+    println!("{:?}", solve_board(&["***"]));
+    println!("{:#?}", solve_board(&["   ", " * ", "   ",]));
+    println!("{:#?}", solve_board(&["*  ", "   ", "  *",]));
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+[]
+[""]
+["***"]
+[
+    "111",
+    "1*1",
+    "111",
+]
+[
+    "*1 ",
+    "121",
+    " 1*",
+]
+$
+```