document .kops file - does VIPER work

I have the following config file installed and run:

```
$cat $HOME/.kops
KOPS_STATE_STORE=s3://aws.k8spro.com
$ kops toolbox inventory -f in-v1alpha2.yaml
unable to load client set State Store: Required value: Please set the --state flag or export KOPS_STATE_STORE.
A valid value follows the format s3://.
A s3 bucket is required to store cluster state information.

```
And that does not work. We have https://github.com/spf13/viper installed, but I do not know if it is actually wired in correctly ....

Start with the `kops toolbox inventory` command and the reported `$HOME/.kops` configuration. Check how Viper is wired into configuration loading and whether `KOPS_STATE_STORE` is read from that file. Done means documenting the supported configuration method and correcting the example or behavior if needed.

Written by the indexing model from the issue text.