doc: Add nightly feature hint for closures

resolves #767

guide/src/reference/passing-rust-closures-to-js.md

@@ -63,6 +63,12 @@ return but the JavaScript closure still needs to be valid!
 For this scenario, you need the `Closure` type, which is defined in the
 `wasm_bindgen` crate, exported in `wasm_bindgen::prelude`, and represents a
 "long lived" closure.
+The `Closure` type is currently behind a feature which needs to be enabled:
+
+```toml
+[dependencies]
+wasm-bindgen = {version = "^0.2", features = ["nightly"]}
+```
 
 The validity of the JavaScript closure is tied to the lifetime of the `Closure`
 in Rust. **Once a `Closure` is dropped, it will deallocate its internal memory