Added Tectonic to the page.

doc/install/kubernetes/index.md

 # Installing GitLab on Kubernetes
 > Officially supported cloud providers are Google Container Service and Azure Container Service.
 
-> Officially supported schedulers are Kubernetes and Terraform.
+> Officially supported schedulers are Kubernetes, Terraform and Tectonic.
 
 The easiest method to deploy GitLab in [Kubernetes](https://kubernetes.io/) is
 to take advantage of the official GitLab Helm charts. [Helm] is a package