[master] Explain --enable-debug in the contributor guide

doc/devel/contribute.dox

@@ -109,7 +109,9 @@ make distcheck
 
 There are other useful switches which can be passed to configure. It is
 always a good idea to use \c --enable-logger-checks, which does sanity
-checks on logger parameters. If you happen to modify anything in the
+checks on logger parameters. Use \c --enable-debug to enable various
+additional consistency checks that reduce performance but help during
+development. If you happen to modify anything in the
 documentation, use \c --enable-generate-docs. If you are modifying DHCP
 code, you are likely to be interested in enabling the MySQL backend for
 DHCP. Note that if the backend is not enabled, MySQL specific unit-tests