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

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 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.