Add the subject for the exercise `blood_types_s`

A simplified version of `blood_types`

subjects/blood_types_s/README.md

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+## blood_types_s
+
+### Instructions
+
+Use the following table to define the methods asked:
+
+| Blood Types | Donate Blood to  | Receive Blood From |
+|-------------|------------------|--------------------|
+| A+          | A+, AB+          | A+, A-, O+, O-     |
+| O+          | O+, A+, B+, AB+  | O+, O-             |
+| B+          | O+, O-           | B+, B-, O+, O-     |
+| AB+         | AB+              | Everyone           |
+| A-          | A+, A-, AB+, AB- | A-, O-             |
+| O-          | Everyone         | O-                 |
+| B-          | B+, B-, AB+, AB- | B-, O-             |
+| AB-         | AB+, AB-         | AB-, A-, B-, O-    |
+
+Write three methods for BloodType:
+
+- `can_receive_from`: which returns true if `self` can receive blood from `other` blood type
+- `donors`: which returns all the blood types that can give blood to `self`
+- `recipients`: which returns all the blood types that can receive blood from `self`
+
+### Expected Functions and Structures
+
+```rust
+#[derive(Debug, PartialEq, Eq, Clone, PartialOrd, Ord)]
+pub enum Antigen {
+	A,
+	AB,
+	B,
+	O,
+}
+
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone)]
+enum RhFactor {
+	Positive,
+	Negative,
+}
+
+#[derive(PartialEq, Eq, PartialOrd, Ord, Clone)]
+pub struct BloodType {
+	pub antigen: Antigen,
+	pub rh_factor: RhFactor,
+}
+
+impl BloodType {
+	pub fn can_receive_from(&self, other: &Self) -> bool {
+	}
+
+	pub fn donors(&self) -> Vec<Self> {
+	}
+
+	pub fn recipients(&self) -> Vec<Self> {
+}
+```
+
+### Usage
+
+Here is a program to test your function.
+
+```rust
+use blood_types_s::{Antigen, BloodType, RhFactor};
+
+fn main() {
+	let blood_type = BloodType {
+		antigen: Antigen::O,
+		rh_factor: RhFactor::Positive,
+	};
+	println!("recipients of O+ {:?}", blood_type.recipients());
+	println!("donors of O+ {:?}", blood_type.donors());
+	let another_blood_type = BloodType {
+		antigen: Antigen::O,
+		rh_factor: RhFactor::Positive,
+	};
+	println!(
+		"donors of O+ can receive from {:?} {:?}",
+		&another_blood_type,
+		blood_type.can_receive_from(&another_blood_type)
+	);
+}
+```
+
+And its output
+
+```console
+student@ubuntu:~/[[ROOT]]/test$ cargo run
+recipients of O+ [BloodType { antigen: AB, rh_factor: Positive }, BloodType { antigen: O, rh_factor: Positive }, BloodType { antigen: A, rh_factor: Positive }, BloodType { antigen: B, rh_factor: Positive }]
+donors of O+ [BloodType { antigen: O, rh_factor: Positive }, BloodType { antigen: O, rh_factor: Negative }]
+donors of O+ can receive from BloodType { antigen: O, rh_factor: Positive } true
+student@ubuntu:~/[[ROOT]]/test$
+```