docs to explain how the cluster-template can be edited

The workflow explained by cluster-api documentation and in our readme is
the basic one.

But you do not need to use the `clusterctl config cluster` command to
get cluster spec you need to create your workload.

You can write your own one or you can start from the template and modify
it based on your need.

Author: Gianluca Arbezzano

README.md

@@ -92,6 +92,10 @@ This will:
 1. Generate the output
 1. Tell you where it is an the `kubectl apply` command to run
 
+If you want to use your own cluster template or modify the default one
+extensively, [this document](./docs/experiences/custom-cluster-template.md)
+describes how to do so.
+
 ##### Defaults
 
 If you do not change the generated `yaml` files, it will use defaults. You can look in the [templates/cluster-template.yaml](./templates/cluster.yaml) file for details.

docs/experiences/custom-cluster-template.md

@@ -0,0 +1,51 @@
+When using the `clusterctl` you can generate your own cluster spec from a
+template.
+
+This is what happens when you run:
+
+```sh
+$ clusterctl config cluster <cluster-name> --infrastructure packet >
+out/cluster.yaml
+```
+
+The workflow triggered by that command summarized here:
+
+1. Based on the `--infrastructure` option it goes to the right repository and it
+   looks for the `latest` release. At the time I am writing it is
+   [v0.3.0](gh-release-v030).
+2. `clusterctl` lookup a file called `cluster-template.yaml` from the release artifacts
+3. `clusterctl` uses `cluster-template.yaml` plus a set of environment variables
+   that you can find described in the README.md to generate the cluster
+   specification
+4. With the command `> out/cluster.yaml` the generated spec gets moved to
+   `./out/cluster.yaml` other than being printed to `stdout`.
+
+This is good if you do not have particular needs or if you are trying capp.
+
+The current `cluster-templates.yaml` uses `apt` and it depends upon `Ubuntu`.
+But you can open and modify it to match your requirements.
+
+## When should I modify the template?
+
+Every time you feel like the default one is not enough, or if you need more
+automation. Here a few examples:
+
+1. ClusterAPI decided to leave the CNI configuration out from its workflow
+   because there are too many of them. If you want to automate that part and
+   let's suppose you want `flannel` you can add the following line to
+   `postKubeadmCommands` for the `KubeadmControlPlane` resource:
+
+```sh
+kubectl --kubeconfig /etc/kubernetes/admin.conf apply -f https://raw.githubusercontent.com/coreos/flannel/master/Documentation/kube-flannel.yml
+```
+
+2. If you want to use an operating system that is not Ubuntu you can change the
+   `preKubeadmCommands` for the `KubeadmControlPlane` and the
+   `KubeadmConfigTemplate` to use kubernetes binaries or a different package
+   manager.
+
+3. (2a) If you want to change operating system you have to change the `OS` field
+   for the `PacketMachineTemplate` resource
+
+
+[gh-release-v030]: https://github.com/kubernetes-sigs/cluster-api-provider-packet/releases/tag/v0.3.0