added few comments

releng/releng_build_isojigdo.conf.template

@@ -1,8 +1,18 @@
-# *** Set your paths
+# *** Set your paths, see below
 # *** Expected location is $HOME/releng_build_isojigdo.conf
 # *** Then a run of 'releng_build_isojigdo -rc' will pick it up
+
+# xorriso executable to be tested
 XOR=/path/to/xorriso
+
+# Should contain an ISOLINUX setup and some payload files
 DIR=/path/to/input_dir
+
+# Gets created resp. overwritten by this test
 IMG=/path/to/out_image
-ISOLINUX_BIN=/path/to/isolinux.bin
+
+# It is part of the ISOLINUX setup in $DIR
+ISOLINUX_BIN=/path/to/input_dir/isolinux.bin
+
+# Will be created in the ISO image. Should not exist in $DIR
 BOOT_CAT=/path/to/boot.cat