Add install instructions for various K8s vendors/distributions

Gitkube runs on most Kubernetes distributions/vendors as per the install instructions in README. But in some cases, there may be more configuration required to correctly setup for e.g. exposing the gitkubed service may require configuring a Loadbalancer manually.

We need to document the instructions (docs/install.md) covering various vendors like GKE, Rancher, kops based installations, etc. In most cases, the instructions should be default instructions as provided in the README and in other cases the additional steps should be mentioned.

Please use this issue to mention any distribution which is not covered by the installation instructions.