Add exercise drawing for the rust piscine

subjects/drawing/README.md

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+## drawing
+
+### Instructions
+
+- Create a module called geometrical_shapes in another file.
+
+- This module will keeps all the logic for creating and operating with the different geometrical shapes and define two traits `Displayable` and `Drawable`.
+
+- `Drawable` contains the methods `draw` and `color`
+
+- `Displayable` contains the methods `display`.
+
+- Define them in correspondence with the way they're called in the main function
+
+- You have to define the structures for Point, Circle, Line, Rectangle and Triangle and make the code in `main.rs` compile and run.
+
+- You are free to implement all the shapes with the internal structure that you find more adequate, but you have to provide for all the shapes an associated function `new` which will be described next:
+
+- Point: a new point should be created from two i32 values
+- Line: a new line should be created from references to two points also define an associated function called `random` that receives two argument the first is the maximum x value a point can have and the
+second the maximum y value that a point can have
+- Triangle: a new triangle should be created from references to three points
+- Rectangle: a new rectangle should be created from two reference to points
+- Circle: a new circle should be created from a point represented the center and a i32 value representing the radius
+   - also define an associated function called `random` that receives two arguments the first is the maximum x value the center point can have and the second the maximum y value that the center point can have
+
+Don't forget to add the dependencies in your Cargo.toml.
+
+### Usage
+
+```rust
+mod geometrical_shapes;
+
+use geometrical_shapes as gs;
+use gs::{Displayable, Drawable};
+use raster::{Color, Image};
+
+fn main() {
+    let mut image = Image::blank(1000, 1000);
+
+    gs::Line::random(image.width, image.height).draw(&mut image);
+
+    gs::Point::random(image.width, image.height).draw(&mut image);
+
+    let rectangle = gs::Rectangle::new(gs::Point::new(150, 150), gs::Point::new(50, 50));
+    rectangle.draw(&mut image);
+
+    let triangle = gs::Triangle {
+        vertices: (
+            gs::Point::new(500, 500),
+            gs::Point::new(250, 700),
+            gs::Point::new(700, 800),
+        ),
+    };
+    triangle.draw(&mut image);
+
+    for _ in 1..50 {
+        gs::Circle::random(image.width, image.height).draw(&mut image);
+    }
+
+    raster::save(&image, "image.png").unwrap();
+}
+
+impl Displayable for Image {
+    fn display(&mut self, x: i32, y: i32, color: Color) {
+        if x >= 0 && x < self.width && y >= 0 && y < self.height {
+            self.set_pixel(x, y, color).unwrap();
+        }
+    }
+}
+```
+
+### And the expected output is a png file: `image.png`