Add troubleshooting section for result of #370

Closes #370.

docs/TROUBLESHOOTING.md

@@ -7,6 +7,7 @@ If you are experiencing issues with DFW that you don't find represented here, fe
 ---
 
 * [modprobe error when running in Docker](#modprobe-error-when-running-in-docker)
+* [set up rule failed, `DOCKER_OUTPUT`/`DOCKER_POSTROUTING`](#set-up-rule-failed-docker_outputdocker_postrouting)
 
 ---
 
@@ -45,3 +46,17 @@ The important additions over the default run-command are:
 * `--cap-add=SYS_MODULE`
 
     This enables the Docker container to manage the host's kernel modules.
+
+## set up rule failed, `DOCKER_OUTPUT`/`DOCKER_POSTROUTING`
+
+```
+set up rule failed, [-t nat -I DOCKER_OUTPUT -d 127.0.0.11 -p udp --dport 53 -j DNAT --to-destination 127.0.0.11:53982]
+set up rule failed, [-t nat -I DOCKER_POSTROUTING -s 127.0.0.11 -p udp --sport 53982 -j SNAT --to-source :53]
+set up rule failed, [-t nat -I DOCKER_OUTPUT -d 127.0.0.11 -p tcp --dport 53 -j DNAT --to-destination 127.0.0.11:32987]
+set up rule failed, [-t nat -I DOCKER_POSTROUTING -s 127.0.0.11 -p tcp --sport 32987 -j SNAT --to-source :53]
+```
+
+If you have DNS resolution errors and are seeing errors like above, your Kernel might be missing required modules or may be misconfigured.
+You can find detailed information about this in [issue #370].
+
+[issue #370]: https://github.com/pitkley/dfw/issues/370#issuecomment-766272308