bootspec/README: clarify version support

Author: cole-h

bootspec/README.md

@@ -5,3 +5,7 @@ This crate provides various structures and constants useful for interacting with
 See: https://github.com/NixOS/rfcs/pull/125.
 
 The `BootJson` struct implements the `serde::Deserialize` and `serde::Serialize` traits, making it easy to work with existing bootspec documents as well as creating new ones.
+
+## Versioning
+
+* `bootspec` crate versions `1.x` and `2.x` are compatible with [bootspec V1](https://github.com/NixOS/rfcs/pull/125).