Better background on using registries

[jdolitsky]

see https://github.com/helm/helm-www/pull/623#discussion_r422184389

[phroggyy]

One thing I realised while talking to @hickeyma was that the docs I submitted in #623 are a bit off as the example used there is nginx:1.10, this won't work since nginx is not a helm Chart, so we could update any references I've made to nginx to match the existing registry docs with localhost:5000/myrepo/mychart:2.7.0.

Commenting here just to have a record of it.

[hickeyma]

Yeah, I think the docs could do with improvement especially for people new to registries.