Fix README

[maruina]

While trying the latest version, we got the following error

 Warning  FailedCreatePodSandBox  16m                 kubelet, ip-172-17-224-226.eu-central-1.compute.internal  Failed create pod sandbox: rpc error: code = Unknown desc = [failed to set up sandbox container "45dbad7ae1fb8123ffe3832ea1c007e277006a3cc710a16143fa5b0145c79d01" network for pod "fluentd-splunk-9h7bm": NetworkPlugin cni failed to set up pod "fluentd-splunk-9h7bm_kube-system" network: incompatible CNI versions; config is "0.3.1", plugin supports ["0.4.0"], failed to clean up sandbox container "45dbad7ae1fb8123ffe3832ea1c007e277006a3cc710a16143fa5b0145c79d01" network for pod "fluentd-splunk-9h7bm": NetworkPlugin cni failed to teardown pod "fluentd-splunk-9h7bm_kube-system" network: couldn't discover peer idx from netns /proc/7171/ns/net: failed to lookup "veth0": could not look up "veth0": Link not found]

This is because #83 updated the github.com/containernetworking/cni package. This is setting the current CNI version to 0.4.0 as per https://github.com/containernetworking/cni/blob/v0.7.1/pkg/version/version.go#L28

This PR fix the README with the correct instructions.

Marked as draft because we're testing to see if we need to make any other change.