Document leaf on empty types in README

README.md

@@ -151,6 +151,20 @@ The `#[rust_sitter::leaf(...)]` annotation can be used to define a leaf node in
 - the `text` parameter takes a string that is used to match the text of the leaf node. This parameter is mutually exclusive with `pattern`.
 - the `transform` parameter takes a function that is used to transform the matched text (an `&str`) into the desired type. This parameter is optional if the target type is `()`.
 
+`leaf` can either be applied to a field in a struct / enum variant (as seen above), or directly on a type with no fields:
+
+```rust
+#[rust_sitter::leaf(text = "9")]
+struct BigDigit;
+
+enum SmallDigit {
+    #[rust_sitter::leaf(text = "0")]
+    Zero,
+    #[rust_sitter::leaf(text = "1")]
+    One,
+}
+```
+
 ### `#[rust_sitter::prec(...)]` / `#[rust_sitter::prec_left(...)]` / `#[rust_sitter::prec_right(...)]`
 This annotation can be used to define a non/left/right-associative operator. This annotation takes a single parameter, which is the precedence level of the operator (higher binds more tightly).