Custom README to describe the Emscripten work-in-progress
Use {STDIN,STDOUT}_FILENO instead of 0, 1

Instead of referring to I/O streams (stdin, stdout) by their integer
file descriptors (0, 1), use the respective preprocessor symbols
(STDIN_FILENO, STDOUT_FILENO).

Originally suggested by user HostileFork on StackOverflow Chat.