Some documentation on the upload process

This is a preliminary work to #41 (closed) in order to better understand how the importuploads cronjob is moving files around.

This work are put in light two new issues:

• Files uploaded under /pub FTP-wise are never checked for stalling (and thus never cleaned-up) #62 (closed)
• Files uploaded directly under /pub FTP-wise are never moved to the processing queue. #61 (closed)