Add Prefix Path documentation

content/rke/latest/en/config-options/_index.md

@@ -18,6 +18,7 @@ There are several options that can be configured in cluster configuration option
 ### Configuring Kubernetes Cluster
 * [Cluster Name](#cluster-name)
 * [Kubernetes Version](#kubernetes-version)
+* [Prefix Path](#prefix-path)
 * [System Images]({{< baseurl >}}/rke/latest/en/config-options/system-images/)
 * [Services]({{< baseurl >}}/rke/latest/en/config-options/services/)
 * [Extra Args and Binds and Environment Variables]({{< baseurl >}}/rke/latest/en/config-options/services/services-extras/)
@@ -89,6 +90,19 @@ Prior to v0.2.0, if a version is defined in `kubernetes_version` and is not foun
 
 If you want to use a different version from the supported list, please use the [system images]({{< baseurl >}}/rke/latest/en/config-options/system-images/) option.
 
+### Prefix Path
+
+For some operating systems including ROS, and CoreOS, RKE stores its resources to a different prefix path, this prefix path is by default for these operating systems is:
+```
+/opt/rke
+```
+So `/etc/kubernetes` will be stored in `/opt/rke/etc/kubernetes` and `/var/lib/etcd` will be stored in `/opt/rke/var/lib/etcd` etc.
+
+To change the default prefix path for any cluster, you can use the following option in the cluster configuration file `cluster.yml`:
+```
+prefix_path: /opt/custom_path
+```
+
 ### Cluster Level SSH Key Path
 
 RKE connects to host(s) using `ssh`. Typically, each node will have an independent path for each ssh key, i.e. `ssh_key_path`, in the `nodes` section, but if you have a SSH key that is able to access **all** hosts in your cluster configuration file, you can set the path to that ssh key at the top level. Otherwise, you would set the ssh key path in the [nodes]({{< baseurl >}}/rke/latest/en/config-options/nodes/).