Update readme for yarn compatibility

README.md

@@ -2,7 +2,7 @@
 
 **Upgrading the plugin? See [UPGRADING.md](./UPGRADING.md)**
 
-Generates a [Nix] expression to build a [Yarn] v3 project (not using
+Generates a [Nix] expression to build a [Yarn] v3 or v4 project (not using
 zero-install).
 
 - Provides a `yarn` shell alias in the Nix builder — no global Yarn v1 install
@@ -43,8 +43,8 @@ Related projects:
 
 ## Usage
 
-The minimum version of Yarn is 3.1.0. Run the following in your project folder
-to check:
+This plugin is compatible with Yarn >= 3.1.0 or >= 4.0.0. Run the following in
+your project folder to check:
 
 ```sh
 # Check your Yarn version