Include CLI instructions in scale-your-cluster.md (#1791)

* Update scale-your-cluster.md

* Improve UCP CLI instructions

datacenter/ucp/2.1/guides/admin/configure/scale-your-cluster.md

@@ -102,6 +102,46 @@ If you're load-balancing user requests to UCP across multiple manager nodes,
 when demoting those nodes into workers, don't forget to remove them from your
 load-balancing pool.
 
+## Scale your cluster from the CLI
+
+You can also use the command line to do all of the above operations. To get the
+join token, run the following command on a manager node:
+
+```none
+$ docker swarm join-token worker
+```
+
+If you want to add a new manager node instead of a worker node, use
+`docker swarm join-token manager` instead. If you want to use a custom listen
+address, add the `--listen-addr` arg:
+
+```none
+docker swarm join \
+    --token SWMTKN-1-2o5ra9t7022neymg4u15f3jjfh0qh3yof817nunoioxa9i7lsp-dkmt01ebwp2m0wce1u31h6lmj \
+    --listen-addr 234.234.234.234 \
+    192.168.99.100:2377
+```
+
+Once your node is added, you can see it by running `docker node ls` on a manager:
+
+```none
+$ docker node ls
+```
+
+To change the node's availability, use:
+
+```
+$ docker node update --availability drain node2
+```
+
+You can set the availability to `active`, `pause`, or `drain`.
+
+To remove the node, use:
+
+```
+$ docker node rm <node-hostname>
+```
+
 ## Where to go next
 
 * [Use your own TLS certificates](use-your-own-tls-certificates.md)