Add logging instructions to unbound guide (#354)

* Add logging instructions to unbound guide

* Remove leftovers

* Remove copy&paste error

* Fix markdownlint errors

* Use --parent option with mkdir

Signed-off-by: yubiuser <ckoenig@posteo.de>

Co-authored-by: Dan Schaper <dan.schaper@pi-hole.net>

docs/guides/unbound.md

@@ -157,3 +157,43 @@ Finally, configure Pi-hole to use your recursive DNS server by specifying `127.0
 ![Upstream DNS Servers Configuration](../images/RecursiveResolver.png)
 
 (don't forget to hit Return or click on `Save`)
+
+### Add logging to unbound
+
+!!! warning
+    It's not recommonded to increase verbosity for daily use, as unbound logs a lot. But it might be helpful for debugging purposes.
+
+There are five levels of verbosity
+
+```
+Level 0 means no verbosity, only errors
+Level 1 gives operational information
+Level 2 gives  detailed operational  information
+Level 3 gives query level information
+Level 4 gives  algorithm  level  information
+Level 5 logs client identification for cache misses
+```
+
+First, specify the log file and the verbosity level in the `server` part of
+`/etc/unbound/unbound.conf.d/pi-hole.conf`:
+
+```ini
+server:
+    # If no logfile is specified, syslog is used
+    logfile: "/var/log/unbound/unbound.log"
+    verbosity: 1
+```
+
+Second, create log dir and file, set permissions:
+
+```
+sudo mkdir -p /var/log/unbound
+sudo touch /var/log/unbound/unbound.log
+sudo chown unbound /var/log/unbound/unbound.log
+```
+
+Third, restart unbound:
+
+```
+sudo service unbound restart
+```