feat(dres_code): add new rust exercise

subjects/dress_code/README.md

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+## dress_code
+
+### Instructions
+
+Create a function called `choose_outfit` that receives the following input:
+- a `formality_level` as an `Option<u32>`;
+- an `invitation_message` as a `Result<&str>`.
+
+The function will return a struct `Outfit` which contains: 
+- `jacket`, an `enum` `Jacket` that contains `Black`, `White` and `Flowers`;
+- 'hat', an `enum` `Hat` that contains `Snapback`, `Baseball`, `Fedora`.
+
+```rust
+#[derive(Debug, PartialEq, Eq)]
+pub struct Outfit {
+    pub jacket: Jacket,
+    pub hat: Hat,
+}
+```
+
+For the `jacket`:
+- the jacket should be `Flowers` when the `formality_level` is unset;  
+- the jacket should be `White` when the `formality_level` is more than 0;
+- otherwise it should be `Black`.
+
+For the `hat`:
+- if the `invitation_message` is `Ok()` it should be `Fedora`;
+- otherwise it should be `Snapback`.
+
+In the specific case where `formality_level` is `None` and `invitation_message`
+is not `Ok()` then the `jacket` should be `Flowers` and the `hat` should be `Baseball`.
+
+Remember that all the `enum` and `struct` used must be `pub`. 
+
+### Expected functions
+
+```rust
+pub fn choose_outfit(formality_level: Option<u32>, invitation_message: Result<&str>) -> Outfit {}
+```
+
+### Usage
+
+Here is a program to test your function.
+
+```rust
+use dress_code::*;
+
+fn main() {
+    println!("My outfit will be: {:?}", choose_outfit(Some(0), Ok("Dear friend, ...")));
+}
+```
+
+And its output:
+```console
+$ cargo run
+My outfit will be: Outfit { jacket: Black, hat: Fedora }
+...
+
+```
+
+### Notions
+
+- [patterns](https://doc.rust-lang.org/book/ch18-00-patterns.html)