Update README.md

[haikusw]

The command line instructions didn't work as written. Updated to make them work.

That said, I'm getting an error when I open Icarus/Icarus.novaextension after building and executing the updated commands:

• build the DebugAdaptor:

cd DebugAdaptor
swift build --product LLDBAdapter --configuration release
cp .build/release/LLDBAdapter ../Icarus.novaextension/Executables/LLDBAdaptor

But I think this may be because Icarus uses:

function debugAdapterPath() {
    let adapterPath = nova.path.normalize(nova.path.join(nova.extension.path, "Executables/LLDBAdapter"));

    // Check adapter executability.
    if (!nova.fs.access(adapterPath, nova.fs.constants.X_OK)) {
        // Set +x on the adapter to get around an issue with extensions being installed by Nova.
        nova.fs.chmod(adapterPath, 0o755);
    }

    return adapterPath;
}

which maybe doesn't work for an extension project activated via "Activate Project As Extension" in the "Extensions" menu?