Added some basic notes on choosing the right kind of DNS record for Porter-provisioned K8s load balancers.

docs/guides/https-and-custom-domains.md

@@ -1,5 +1,8 @@
 Porter secures application endpoints with HTTPS and sets up custom domains using [cert-manager](https://cert-manager.io/) and [lets-encrypt](https://letsencrypt.org/). Below are the steps to set up custom domains on each cloud provider.
 
+# Choosing between `A` and `CNAME` records
+A basic rule of thumb you can follow whilst trying to choose between setting up an `A` records as opposed to a `CNAME` record for your cluster, is to see how your cluster's load balancer is exposed to the Internet. If your load balancer exposes a public IP, you should use an `A` record for your custom domain that points to the public IP - as is the case with GKE. If your load balancer exposes a FQDN, then you should use a `CNAME` record - this is common with EKS clusters that use AWS Network Load Balancers/Application Load Balancers.
+
 # Amazon Web Services (AWS)
 
 Porter provisions a EKS cluster and an ECR registry in your AWS account by default. Along with these resources, it also deploys both the `nginx-ingress` controller and cert-manager on the provisioned cluster - there is no need to separately install these components.