feat(matrix_multiplication): add new rust exam exercise

subjects/matrix_multiplication/README.md

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+## matrix_multiplication
+
+### Instructions
+
+- Define a `struct` named `Matrix` as a tuple of 2 tuples. The nested tuple will contain 2 `i32`s.
+
+- Create a **function** named `multiply` that receives a Matrix and an i32 and returns the Matrix with each number multiplied by the second argument. 
+```rust
+pub fn multiply(m: Matrix, val: i32) -> Matrix {
+}
+```
+
+`Matrix` must implement `Debug`, `PartialEq` and `Eq`. You can use `derive`.
+
+> Remember that you are defining a library, so any element that can be called from an external crate must be made public.
+
+### Usage
+
+Here is a possible program to test your function
+
+```rust
+use matrix_multiplication::multiply;
+use matrix_multiplication::Matrix;
+
+fn main() {
+    let matrix = Matrix((1, 3), (4, 5));
+    println!("Original matrix {:?}", matrix);
+    println!("Matrix after multiply {:?}", multiply(matrix, 3));
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+Original matrix Matrix ((1, 3), (4, 5))
+Matrix after multiply ((3, 9), (12, 15))
+$
+```
+
+### Notions
+
+- [Defining a struct](https://doc.rust-lang.org/stable/book/ch05-01-defining-structs.html)
+
+- [The Tuple Type](https://doc.rust-lang.org/stable/book/ch03-02-data-types.html)
+
+- [Tuples](https://doc.rust-lang.org/rust-by-example/primitives/tuples.html)
+
+- [Tuple Structs without Named Fields](https://doc.rust-lang.org/stable/book/ch05-01-defining-structs.html)
+
+- [Adding Useful Functionality with Derived Traits](https://doc.rust-lang.org/stable/book/ch05-02-example-structs.html)
+
+- [Chapter 7](https://doc.rust-lang.org/stable/book/ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html)