docs: add section about multi-master setups

docs/usage/multimaster.md

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+# Creating multi-master clusters
+
+!!! note "Important note"
+    For the best results (and less unexpected issues), choose 1, 3, 5, ... master nodes.
+
+## Embedded dqlite
+
+Create a cluster with 3 master nodes using k3s' embedded dqlite database.
+The first master to be created will use the `--cluster-init` flag and k3d will wait for it to be up and running before creating (and connecting) the other master nodes.
+
+```bash
+    k3d create cluster multimaser --masters 3
+```
+
+## Adding master nodes to a running cluster
+
+In theory (and also in practice in most cases), this is as easy as executing the following command:
+
+```bash
+    k3d create node newmaster --cluster multimaster --role master
+```
+
+!!! note "There's a trap!"
+    If your cluster was initially created with only a single master node, then this will fail.
+    That's because the initial master node was not started with the `--cluster-init` flag and thus is not using the dqlite backend.