Output to STDERR

doc/faqs/how-to-install-composer-programmatically.md

@@ -20,7 +20,7 @@ then
     rm composer-setup.php
     exit $RESULT
 else
-    echo 'ERROR: Invalid installer signature'
+    >&2 echo 'ERROR: Invalid installer signature'
     rm composer-setup.php
     exit 1
 fi