add a note that repo will be archived

[rimusz]

replace Helm Classic (the helm/helm repository) is no longer actively developed but will remain available until kubernetes/helm has stabilized. with Helm Classic (the helm/helm repository) is no longer developed and this repo will be archived by end of June

same for charts.

[mboersma]

👍 We should just wait to get consensus from other Helm teamers before creating an archival notice.