Document TLS cert generation for tests

[jdolitsky]

Maybe add a file BUILDING.md with instructions documented here: https://github.com/chartmuseum/helm-push/pull/44#issue-350434957

[karuppiah7890]

@jdolitsky I noticed here that we are using names that are confusing

https://github.com/chartmuseum/helm-push/blob/master/cmd/helmpush/main_test.go#L168

https://github.com/chartmuseum/helm-push/blob/master/cmd/helmpush/main_test.go#L174

Can we use better names? Say, "client ca" when it's actually a client CA and Client cert when it's a Client cert and the same for server CA and certificates

[karuppiah7890]

Currently it's mixed up - the name looks like it's based on where the item is present, or where the item is being sent to / who it is checked by, instead of who it belongs to