update kind/ceos.md doc re: cgroup v1 requirement

reference issue: #467.  ceos-lab only supports cgroup v1 operation.
this advises users taht they may need to revert their runtime
environment to accomodate this requirement.

docs/manual/kinds/ceos.md

@@ -103,6 +103,10 @@ topology:
     - endpoints: ["ceos_rtr1:et2", "ceos_rtr3:et1"]
 ```
 
+## cgroup requirements
+
+As of this writing (22-June, 2021), ceos-lab images require a cgroup v1 environment.  For many users this should not require any changes to the runtime environment.  However, some users (ref: [#467](https://github.com/srl-labs/containerlab/issues/467)) will potentially need to configure their container runtime environment to utilize a cgroup v1 environment.  Other cgroup v1-based software (e.g.: Kubernetes) requires a cgroup v1-based runtiem environment as well.  Consult your distribution's documentation for details regarding configuring cgroup v1 operation. 
+
 ## Features and options
 ### Node configuration
 cEOS nodes have a dedicated [`config`](../conf-artifacts.md#identifying-a-lab-directory) directory that is used to persist the configuration of the node. It is possible to launch nodes of `ceos` kind with a basic config or to provide a custom config file that will be