Ansible 2.8.x note (#4908)

Update README.md to link to the open issue that shows Ansible 2.8.x doesn't work with Kubespray.  The requirements.txt file is already fixed to 2.7.8 so only the README needed updating, I think.

README.md

@@ -135,7 +135,7 @@ Note: The list of validated [docker versions](https://github.com/kubernetes/kube
 Requirements
 ------------
 -   **Minimum required version of Kubernetes is v1.13**
--   **Ansible v2.7.8 (or newer) and python-netaddr is installed on the machine
+-   **Ansible v2.7.8 (or newer, but [not 2.8.x](https://github.com/kubernetes-sigs/kubespray/issues/4778)) and python-netaddr is installed on the machine
     that will run Ansible commands**
 -   **Jinja 2.9 (or newer) is required to run the Ansible Playbooks**
 -   The target servers must have **access to the Internet** in order to pull docker images. Otherwise, additional configuration is required (See [Offline Environment](https://github.com/kubernetes-sigs/kubespray/blob/master/docs/downloads.md#offline-environment))