Add the subject for the exercise `flat_tree`

subjects/flat_tree/README.md

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+## flat_rust
+
+### Instructions
+
+- Define the functions `flatten_tree` that receives a std::collections::BTreeSet and returns a new `Vec` with the elements in the binary tree in order.
+
+### Expected function
+
+```rust
+pub fn flatten_tree<T: ToOwned<Owned = T>>(tree: &BTreeSet<T>) -> Vec<T> {
+}
+```
+
+### Usage
+
+Here is a possible test for your function:
+
+```rust
+fn main() {
+	let mut tree = BTreeSet::new();
+	tree.insert(34);
+	tree.insert(0);
+	tree.insert(9);
+	tree.insert(30);
+	println!("{:?}", flatten_tree(&tree));
+
+	let mut tree = BTreeSet::new();
+	tree.insert("Slow");
+	tree.insert("kill");
+	tree.insert("will");
+	tree.insert("Horses");
+	println!("{:?}", flatten_tree(&tree));
+}
+```
+
+And its output:
+
+```console
+student@ubuntu:~/[[ROOT]]/test$ cargo run
+[0, 9, 30, 34]
+["Horses", "Slow", "kill", "will"]
+student@ubuntu:~/[[ROOT]]/test$
+```