Document possible tags for the ansible-playbook

The playbook itself is written to be idempotent, so it doesn't hurt to
run all tasks many times. But, it doesn't need to run all tasks all the
time, therefore you can limit the executional-scope with the documented
tags to only affect certain tasks. This improves the performance a lot!

README.adoc

@@ -102,6 +102,19 @@ To apply the playbook you may need to install additional packages:
 * python3-kubernetes (Debian/Ubuntu)
 --
 
+
+==== Configured tags
+
+init:: Everything needed for the initial setup
+add-server:: Everything needed to add a new https://docs.k3s.io/cli/server[server] to the cluster
+add-agent:: Everything needed to add a new https://docs.k3s.io/cli/agent[agent] to the cluster
+update:: Everything needed to update the cluster
+config:: Everything needed to update the local kube-config
+k8s:: Everything needed to provide the foundational services
+
+[TIP]
+The affected scope of the ansible-playbook can be limited with tags (`--tags tag1,tag2`):
+
 == Enlarge / Reduce size of cluster
 
 Increase::