doc: add rust crate usage example

README.md

@@ -88,7 +88,32 @@ _Coming soon..._
 
 ### Rust
 
-_Coming soon..._
+```toml
+[dependencies]
+smoltok-core = "0.1"
+```
+
+```rust
+use smoltok_core::{RegexBPETokenizerConfig, Tokenizer, Trainable};
+
+fn main() {
+    let text = "hello world hello world hello world hello world";
+    // with default GPT-4 split pattern
+    let config = RegexBPETokenizerConfig::build(512, None).unwrap();
+
+    let tokenizer = config.train(text).unwrap();
+    let tokens = tokenizer.encode("Hello, world!");
+
+    println!("Encoded:");
+    for (i, &token) in tokens.iter().enumerate() {
+        let decoded_token = tokenizer.decode(&[token]).unwrap();
+        println!("- Token {} (ID: {}): {:?}", i, token, decoded_token);
+    }
+
+    let decoded = tokenizer.decode(tokens.as_slice()).unwrap();
+    println!("\nFull decoded text: {:#?}", decoded);
+}
+```
 
 ## Exercise