website: add PACKER_LOG_PATH docs

website/source/docs/other/debugging.html.markdown

@@ -20,5 +20,10 @@ appear out of order, especially with respect to plugins. In this case,
 it is important to pay attention to the timestamp of the log messages
 to determine order.
 
+In addition to simply enabling the log, you can set `PACKER_LOG_PATH` in order
+to force the log to always go to a specific file when logging is enabled.
+Note that even when `PACKER_LOG_PATH` is set, `PACKER_LOG` must be set in
+order for any logging to be enabled.
+
 If you find a bug with Packer, please include the detailed log by using
 a service such as [gist](http://gist.github.com).