Add Kustomize nameReference documentation for HelmRepository sourceRef/name

[kphunter]

It might help to have some additional documentation about how to teach kustomize to substitute HelmRelease names. For those who use configMap/secretGenerators, adding the following section to a kustomizeconfig.yaml would keep the HelmRelease and HelmRepository in sync:

nameReference:
- kind: HelmRepository
  fieldSpecs:
  - path: spec/chart/spec/sourceRef/name
    kind: HelmRelease

Seems like this may have been part of the issue here: fluxcd/flux2#445.