GKE authentication is underdocumented

[x-yuri]

It's a documentation issue, so I believe the usual template doesn't apply here.

The methods described in the documentation:

• the implicit way: I bet it's very unusual to run terraform inside a cluster
• 2 ways where you need to reference ~/.kube: I don't want to run any commands before running terraform, moreover I need to involve myself with contexts in case I have more than 1 cluster
• the exec plugin way: no example for GKE, and I thought it's not possible until I stumbled into this

I believe the way described by the last link is what should be recommended. But it's like not at all obvious where it's documented. At least it deserves a link form the hashicorp/helm documentation.

[sheneska]

Hi @x-yuri, thanks for the suggestion. We've added it!