doc: Add documentation on MTU settings for Neutron.

JIRA:OSPC-610

docs/openstack-compute-kit-neutron.md

@@ -96,3 +96,40 @@ sed 's/your.domain.tld/<YOUR_DOMAIN>/g' \
 ``` shell
 kubectl --namespace openstack apply -f /etc/genestack/gateway-api/routes/custom-neutron-gateway-route.yaml
 ```
+
+## Neutron MTU settings / Jumbo frames / overlay networks on instances
+
+!!! warning You will likely need to increase the MTU as described here if you want to support creating L3 overlay networks (via any software that creates nested networks, such as _Genestack_ itself, VPN, etc.) on your nova instances. Your physical L2 network will need jumbo frames to support this. You will likely end up with a an MTU of 1280 for overlay networks on instances if you don't, and the abnormally small MTU can cause various problems, perhaps even reaching a size too small for the software to support).
+
+You might want to reference [Neutron documentation on MTU considerations](https://docs.openstack.org/neutron/latest/admin/config-mtu.html)
+
+As an example of changing some values of interest, in a file for your Neutron
+Helm overrides, you can use a stanza like:
+
+```
+conf:
+  neutron:
+    DEFAULT:
+      global_physnet_mtu: 9000
+  plugins:
+    ml2_conf:
+      ml2:
+        path_mtu: 4000
+        physical_network_mtus: physnet1:1500
+```
+
+(You can see the Neutron helm overrides file in the installation command above
+as `-f /opt/genestack/base-helm-configs/neutron/neutron-helm-overrides.yaml`,
+but you can supply this information with a second `-f` switch in a separate
+overrides file for your environment if desired. If so, place your second
+`-f` after the first.)
+
+With settings in the example, physical networks get a default MTU of 9000 in
+`global_physnet_mtu`. You can override this for specific networks in
+`physical_network_mtus`, which shows `physnet1` with an MTU of 1500 here,
+which handles public Internet traffic in this case, which shouldn't get jumbo
+frames.
+
+`path_mtu` sets the MTU for tenant or project networks. For `path_mtu` 4000 in
+the example, nova instances will get an MTU of 3942 after 58 bytes of overhead.
+