doc: clarify DEBUG usage macro

Signed-off-by: Alexey Perevalov <a.perevalov@samsung.com>
Signed-off-by: Pablo Neira Ayuso <pablo@gnumonks.org>
diff --git a/INSTALL b/INSTALL
index e17e844..d62b428 100644
--- a/INSTALL
+++ b/INSTALL
@@ -70,6 +70,8 @@
 (-O0 is used to turn off instruction reordering, which makes debugging
 much easier.)
 
+To show debug traces you can add -DDEBUG to CFLAGS option
+
 
 Other notes
 ===========