Update docs to encourage people to *close* their file handles

[sshaw]

I've come across several cases where people were using this library like this:

S3Object.store(file, open(file), 'jukebox')

These were all in long-running processes, sadly.

My conclusion to this madness is that people are copying and pasting what is the README and modifying it to suite their needs. These changes update the README to give them something to copy and paste that won't result in EMFILE, ENFILE, or something else.