Add documentation how to add new OS/k8s version combinations

Author: deitch

README.md

@@ -208,17 +208,23 @@ To see the name of the docker image that would be built, run:
 make image-name
 ```
 
+To push it out:
+
+```
+make docker-push
+```
+
 To build individual components, call its target:
 
 ```
+make manifests
 make manager
 make clusterctl
-make manifests
 ```
 
 As always with `make`, you can force the rebuilding of a component with `make -B <target>`.
 
-## Running locally
+## Running manager locally
 
 You can run the `manager` locally on your own laptop in order to ease and speed development, or even run it through a debugger. The steps are:
 
@@ -236,6 +242,26 @@ In the above example:
 
 Caching the CA caches the certs **and the keys**. Only do this in test mode, or if you really are sure what you are doing. The purpose, in this case, is to allow you to stop and start the process, and pick up existing certs.
 
+## Supported OS and Versions
+
+CAPP (Cluster API Provider for Packet) supports Ubuntu 18.04 and Kubernetes 1.14.3. To extend it to work with different combinations, you only need to edit the file [config/default/machine_configs.yaml](./config/default/machine_configs.yaml).
+
+In this file, each list entry represents a combination of OS and Kubernetes version supported. Each entry is composed of the following parts:
+
+* `machineParams`: list of the combination of OS image, e.g. `ubuntu_18_04`, and Kubernetes versions, both control plane and kubelet, to install. Also includes the container runtime to install.
+* `userdata`: the actual userdata that will be run on server instance startup.
+
+When trying to install a new machine, the logic is as follows:
+
+1. Take the requested image and kubernetes versions.
+1. Match those to an entry in `machineParams`. If it matches, use this `userdata`.
+
+Important notes:
+
+* There can be multiple `machineParams` entries for each `userdata`, enabling one userdata script to be used for more than one combination of OS and Kubernetes versions.
+* There are versions both for `controlPlane` and `kubelet`. `master` servers will match both `controlPlane` and `kubelet`; worker nodes will have no `controlPlane` entry.
+* The `containerRuntime` is installed as is. The value of `containerRuntime` will be passed to the userdata script as `${CR_PACKAGE}`, to be installed as desired.
+
 ## References
 
 * [kubeadm yaml api](https://godoc.org/k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm/v1beta2)