feat(macro_map): new optional exercise for rust piscine

subjects/macro_map/README.md

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+## macro_map
+
+### Instructions
+
+Create a macro rule called `hash_map` to initialize and declare an `HashMap` at the same time, very similar to what `vec!` macro does for `Vector`.
+
+> Your macro should accept both leading commas and non leading commas syntax to be more flexible in terms of coding style and reflect the language general style.
+
+### Expected Macro
+
+```rust
+macro_rules! hash_map {
+}
+```
+
+### Usage
+
+Here is a possible program to test your function,
+
+```rust
+use macro_map::hash_map;
+use std::collections::HashMap;
+
+fn main() {
+    let empty: HashMap<u32, u32> = hash_map!();
+    let new = hash_map!('a' => 22, 'b' => 1, 'c' => 10);
+    let nested = hash_map!(
+        "first" => hash_map!(
+            "Rob" => 32.2,
+            "Gen" => 44.1,
+            "Chris" => 10.,
+        ),
+        "second" => hash_map!()
+    );
+    println!("{:?}", empty);
+    println!("{:?}", new);
+    println!("{:?}", nested);
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+{}
+{'b': 1, 'a': 22, 'c': 10}
+{"first": {"Rob": 32.2, "Gen": 44.1, "Chris": 10.0}, "second": {}}
+$
+```
+