message: add k8s constraints to docs
diff: ## Requirements&#x20;
  -* Kubernetes 1.18+
  +* Kubernetes 1.19+ (Kubernetes 1.22+ is not yet supported, see https://github.com/uselagoon/lagoon/issues/2816 for progress)
  * Familiarity with [Helm](https://helm.sh) and [Helm Charts](https://helm.sh/docs/topics/charts/#helm), and [kubectl](https://kubernetes.io/docs/tasks/tools/).
  * Ingress controller (ingress-nginx)
  * Cert manager (for TLS) - We highly recommend using letsencrypt
  