diff --git a/README.md b/README.md
index 428b3361..d154a684 100644
--- a/README.md
+++ b/README.md
@@ -48,6 +48,7 @@ v.sort();
 
 By default, SmallVec does not make use of any feature. SmallVec without any features enabled does not make use of the standard library.
 
+- `allocator-api2` (default enabled, nightly if disabled): uses the `allocator-api2` backend for the allocator API, otherwise it uses unstable alloc traits
 - `arbitrary`: implements `Arbitrary` for any `SmallVec` storing elements that implement `Arbitrary`
 - `borsh`: implements `BorshSerialize`, `BorshDeserialize` and `BorshSchema`
 - `bytes`: implements `BufMut` for SmallVec