added new callout for subnet machine pool

Author: fmcdonal

modules/creating-a-machine-pool-cli.adoc

@@ -33,6 +33,7 @@ ifdef::openshift-rosa[]
                           --disk-size=<disk_size> <8>
                           --availability-zone=<availability_zone_name> <9>
                           --additional-security-group-ids <sec_group_id> <10>
+                          --subnet string <11>
 
 endif::openshift-rosa[]
 ----
@@ -66,6 +67,25 @@ For fault-tolerant worker machine pools, choosing a Multi-AZ machine pool distri
 * A Single-AZ machine pool with one availability zone can have a machine count in multiples of 1, such as 1,2,3,4 and so on.
 ====
 <10> Optional: For machine pools in clusters that do not have Red Hat managed VPCs, you can select additional custom security groups to use in your machine pools. You must have already created the security groups and associated them with the VPC that you selected for this cluster. You cannot add or edit security groups after you create the machine pool. For more information, see the requirements for security groups in the "Additional resources" section.
+<11> Optional: For BYO VPC clusters, you can select a subnet to create a Single-AZ machine pool.
+If the subnet is out of your cluster creation subnets, there must be a tag with a key `kubernetes.io/cluster/<infra-id>` and value `shared`.
+Customers can obtain the Infra ID by using the following command:
++
+[source,terminal]
+----
+$ rosa describe cluster -c <cluster name>|grep "Infra ID:"
+----
++
+.Example output
+[source,terminal]
+----
+Infra ID:                   mycluster-xqvj7
+----
++
+[NOTE]
+====
+You cannot set both `--subnet` and `--availability-zone` at the same time, only 1 is allowed for a Single-AZ machine pool creation.
+====
 endif::openshift-rosa[]
 --
 +