Use helm archive instead of Kubernetes stable charts repo

Changelog for 0.6.28:

Commits
14c766b Update remaining apps to helm chart mirror by @alexellis
cab943b Move to charts.helm.sh for docker-registry by @alexellis
afb3701 Update README.md by @alexellis
ac0f475 Update headings by @alexellis
229f96f Add sponsorship link by @alexellis
e6645dd Add Venafi to the sponsored apps list by @alexellis

Generated by Derek