Document how to join a cluster to a namespaced federation
@@ -351,6 +351,27 @@ on both the host and member clusters.
It may be useful to supply `FEDERATION_NAMESPACE=test-namespace` to
allow the examples to work unmodified.
+### Joining Clusters
+
+Joining additional clusters to a namespaced federation requires
+providing additional arguments to `kubefed2 join`:
+
+- `--federation-namespace=<namespace>` to ensure the cluster is joined
+ to the federation running in the specified namespace
+- `--registry-namespace=<namespace>` if using `--add-to-registry` to
+ ensure the cluster is registered in the correct namespace
+- `--limited-scope=true` to ensure that the scope of the service account created in
+ the target cluster is limited to the federation namespace
+
+To join `mycluster` when `FEDERATION_NAMESPACE=test-namespace` was used for deployment:
+
+```bash
+./bin/kubefed2 join mycluster --cluster-context mycluster \
+ --host-cluster-context mycluster --add-to-registry --v=2 \
+ --federation-namespace=test-namespace --registry-namespace=test-namespace \
+ --limited-scope=true
+```
+
### Deployment Cleanup
Cleanup similarly requires the use of the same environment variables
