Document crate features in Readme

Author: phil-opp

README.md

@@ -28,5 +28,14 @@ pub fn init_heap() {
 }
 ```
 
+## Features
+
+- **`use_spin`** (default): Provide a `LockedHeap` type that implements the [`GlobalAlloc`] trait by using a spinlock.
+- **`alloc_ref`**: Provide an implementation of the unstable [`AllocRef`] trait.
+    - Warning: The `AllocRef` trait is still regularly changed on the Rust side, so expect some regular breakage when using this feature.
+
+[`GlobalAlloc`]: https://doc.rust-lang.org/nightly/core/alloc/trait.GlobalAlloc.html
+[`AllocRef`]: https://doc.rust-lang.org/nightly/core/alloc/trait.AllocRef.html
+
 ## License
 This crate is dual-licensed under MIT or the Apache License (Version 2.0). See LICENSE-APACHE and LICENSE-MIT for details.