docs(smallest): add readme with description of subject

subjects/smallest/README.md

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+## smallest
+
+### Instructions
+
+Create a function named `smallest` that gets the smallest positive number in the `HashMap`.
+
+### Expected Function
+
+```rust
+pub fn smallest(h: HashMap<&str, i32>) -> i32 {
+}
+```
+
+### Usage
+
+Here is a program to test your function.
+
+```rust
+use std::collections::HashMap;
+use smallest::smallest;
+
+fn main() {
+
+    let mut hash = HashMap::new();
+    hash.insert("Cat", 122);
+    hash.insert("Dog", 333);
+    hash.insert("Elephant", 334);
+    hash.insert("Gorilla", 14);
+
+    println!("The smallest of the elements in the HashMap is {}", smallest(hash));
+}
+```
+
+And its output
+
+```console
+$ cargo run
+The smallest of the elements in the HashMap is 14
+$
+```
+
+### Notions
+
+- [hash maps](https://doc.rust-lang.org/book/ch08-03-hash-maps.html)