The base64 data from the decoder to the file writer is in binary, using only the "w" flag would work in most unix systems but its incorrect and for all systems with crazy line_feed/Carrier_return or any character conversion the right way to save a binary data is using the "wb" flag.
Check for core feature `httpfetch_binary_data` to run the updater properly.
This requires Minetest cb00632 or newer.
README.md: Add documentation for the updater
