Clarify the use of buffers in the write-up

[NicMcPhee]

A number of people didn't understand the expectation that we should be using buffered reading and writing (things like fread and fwrite). This suggests that the write-up could be clarified or (and I'm leaning to this) that the pre-lab could be expanded/clarified to make that a bigger deal there so they're more ready for the buffer-oriented file I/O when the lab starts.