Put the SD-WAN images into their own directory.

Also, add a README for the SD-WAN images.

node-definitions/cisco-sd-wan/README.md

@@ -0,0 +1,21 @@
+# Cisco SD-WAN Node Definitions
+
+This directory contains the following node definitions:
+
+* `iosxe-sdwan.yaml` - Cisco cEdge CSR1000v node definition
+* `viptela-bond.yaml` - Cisco vBond node definition
+* `viptela-edge.yaml` - Cisco vEdge node definition
+* `viptela-manage.yaml` - Cisco vManage node definition
+* `viptela-smart.yaml` - Cisco vSmart node definition
+
+
+
+### Image Availability
+
+Images can be downloaded from https://software.cisco.com/download/home/286320995/type with a proper Cisco.com account and entitlement.
+
+
+
+### Known Issues
+
+While the `viptela-*` nodes support cloud-init day 0 configuration, the config extraction does not re-generate the cloud-init config.  Therefore, config extraction will not work correctly on the `viptela-*` nodes.