doc, guix: Include arm64-apple-darwin into codesigned archs

Author: hebasto

contrib/guix/README.md

@@ -75,7 +75,7 @@ crucial differences:
 
 1. Since only Windows and macOS build outputs require codesigning, the `HOSTS`
    environment variable will have a sane default value of `x86_64-w64-mingw32
-   x86_64-apple-darwin` instead of all the platforms.
+   x86_64-apple-darwin arm64-apple-darwin` instead of all the platforms.
 2. The `guix-codesign` command ***requires*** a `DETACHED_SIGS_REPO` flag.
     * _**DETACHED_SIGS_REPO**_