diff --git a/HACKING b/HACKING
index 1b9e848..737f9c6 100644
--- a/HACKING
+++ b/HACKING
@@ -11,6 +11,8 @@ Here are some directions to get you started:
      is consistent in style and thus easier to read.
      Look around and respect the same style.
      Don't use tabs. Increment unit is two spaces.
+     Don't leave dandling spaces or tabs at EOL. Helper script:
+     $ find . -name '*.[ch]' -exec perl -pi -e 's/[ \t]+$//;' {} \;
 
   2. Chase warnings: no warning should be introduced by your changes
   Depending what you touch, you can check with: