Added a note about a debug setting

README.md

@@ -94,6 +94,13 @@ Which one should I choose?
 > Performance difference between Redis and Memcached is generally quite low, so your setup
 > should normally depend more on the configuration flexibility than on raw speed.
 
+## Debugging
+
+There's a config key `debug` that can be set to `true` for debugging purposes.
+
+It should be set to `false` in production, otherwise log becomes very verbose and might
+contain data that shouldn't normally be logged.
+
 ## Docker [Experimental]
 
 As an experiment, Docker deployment option has been added.