Update 04-extending.md

doc/04-extending.md

@@ -2,6 +2,13 @@
 
 Monolog is fully extensible, allowing you to adapt your logger to your needs.
 
+## Understanding log records
+
+See [the page about log records](message-structure.md) to learn what makes up
+a log record before going further. This is essential to understand as all
+Handlers/Formatters/Processors need to deal with log records in one way or
+another.
+
 ## Writing your own handler
 
 Monolog provides many built-in handlers. But if the one you need does not