Added comment to extract.conf noting the use of an internal plugin for Hilink…

Added comment to extract.conf noting the use of an internal plugin for Hilink encrypted firmware extraction.

src/binwalk/config/extract.conf

@@ -14,6 +14,7 @@
 #   o zlib
 #   o cpio
 #   o Raw LZMA/deflate streams
+#   o Hilink encrypted uImage firmware
 #
 # There are also alternative extractors for the following file formats, implemented as plugins:
 #