README: added Troubleshooting section.

README.md

@@ -926,6 +926,22 @@ submit a pull request in the https://github.com/nginx/unit repository.
 
 ## Troubleshooting
 
-**TBD**
+Unit log for the binary packages is located in `/var/log/unit.log`.
+
+Log file location can also be found by running `unitd --help`.
+
+Debug verbosity level of the log is enabled during configuration time:
+
+```
+./configure --debug
+```
+
+Unit and all modules have to be recompiled and reinstalled after reconfiguring.
+
+Please be aware that the debug log size grows very quickly.
+
+Community mailing list is available at <unit@nginx.org>.
+Subscribe to the mailing list by sending email to <unit-subscribe@nginx.org> or
+at <http://mailman.nginx.org/mailman/listinfo/unit>.
 
 <!-- /section:6 -->