Update HACKING.md

Added some info in debugging section.

[ Squashed: Guide to use configure. --RR ]

doc/HACKING.md

@@ -96,6 +96,8 @@ Here's a list of parts, with notes:
 Debugging
 ---------
 
+You can build c-lightning with DEVELOPER=1 to use dev commands listed in ``cli/lightning-cli help``. ``./configure --enable-developer`` will do that. You can log console messages with log_info() in lightningd and status_trace() in other subdaemons.
+
 You can debug crashing subdaemons with the argument
 `--dev-debugger=lightning_channeld`, where `channeld` is the subdaemon name.
 It will print out (to stderr) a command such as: