Expose nodeSelector to user-facing template

I just deployed on a new resource-constrained K3s cluster running on DigitalOcean and somehow the deployment kept choosing one of the nodes with the least amount of resources available as the deployment target for WordPress. In values.yaml there's an empty object specified for nodeSelector which could potentially be exposed to the values-local.yaml.example such that it's more clear how to target the WordPress installation at a specific node (e.g. one with more resources).

Here's an example in values-local.yaml using nodeSelector:

# This will restrict WordPress deployment to the specified node. This is useful
# when you are working with limited resources and do not want to run WordPress
# on a resourced-constrained node.
#
# See https://kubernetes.io/docs/concepts/scheduling-eviction/assign-pod-node/
# for more information on assigning pods to nodes.
# nodeSelector:
#   kubernetes.io/hostname: k3s-server-1

I'm not sure how often users are using this. But lifting this into the example file may help ease deployments for those who are less familiar with how to accomplish this sort of thing with the current template. If not desirable for the example template, then at least this issue can serve as some documentation for other users.