docs: add docs about ipv6

Resolves #462

Signed-off-by: Balazs Nadasdi <balazs@weave.works>

docs/use_tf_controller/with-ipv6.md

@@ -0,0 +1,16 @@
+# Use TF-Controller with IPv6 addresses.
+
+TF-Controller uses pod IP address to communicate with the runner instance. This
+logic fails when the runner pod has IPv6 address instead of IPv4 as it would try
+to construct a hostname from the IP address.
+
+The TF-Controller has a flag to use pod subdomain resolution instead of an IP
+address, with that enabled the controller will use cluster subdomains and it
+works with IPv6 addresses as the resolution is happening on cluster level.
+
+To enable this feature, you can set `usePodSubdomainResolution` to `true` in the
+Helm values file:
+
+```yaml
+usePodSubdomainResolution: true
+```