Added comment regarding internal extractors to extract.conf

src/binwalk/config/extract.conf

@@ -8,6 +8,8 @@
 # The %% place holder is used when a unique file path is required.
 # For example '%%squashfs-root%%' will be replaced with 'squashfs-root-0'
 # if 'squashfs-root' already exists.
+#
+# Some extractors are internal and not listed here, such as those for zlib files and raw LZMA/deflate streams.
 #################################################################################################################
 
 # Assumes these utilities are installed in $PATH.