README is slightly incomplete

[Miles-Garnsey]

There seems to be a step missing from the README. Enabling workload identity on an existing cluster (here) misses a step mentioned in the Google docs here.

tldr; you need to also enable workload-metadata on the nodepools, only updating the cluster is insufficient.

It'd be great to add this step to the docs since it had me stumped for a couple of days.

[liyihuang]

I had exactly issue today and did a troubleshooting for 2-3 hours to get it resolved. I was about to open this issue but found someone had raise this.