doc: Add documentation on MTU settings for Neutron. (rackerlabs#446)

* doc: Add documentation on MTU settings for Neutron.

JIRA:OSPC-610

docs/openstack-compute-kit-neutron.md

@@ -96,3 +96,38 @@ 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 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).
+
+[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 the 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.