Document supported Kubernetes versions

Zendesk: https://gitlab.zendesk.com/agent/tickets/99249

What versions of Kubernetes do we support? Customer wasn't able to create a needed secret in 1.5 so it appears that version won't work.

Do we have a plan for how we'll manage support for various Kubernetes versions going forward? Both GitLab and Kubernetes projects move quickly so it will be tricky but we should probably have some basic plans for this documented so customers have some idea what will happen.

@bikebilly What do you think?