Remove reference to k8s directory from README

Kolla has moved away from Kubernetes and is now using docker-compose to
define container sets.

Remove last reference to Kubernetes in the README file, and document
the `compose` directory instead.

Change-Id: Ic586da2ae7859817c0da888f6ce5c788188e43d2

README.md

@@ -56,7 +56,7 @@ Directories
 
 * docker - contains artifacts for use with docker build to build appropriate
   images
-* k8s - contains service and pod configuration information for Kubernetes
+* compose - contains the docker-compose files defining the container sets
 * tools - contains different tools for interacting with Kolla
 * devenv - A collection of tools and resources for managing a Kolla
   development environment.