Update README
Submit Assignment

A user can now submit an assignment and it is uploaded, added to the
database, and saved in the users/submissions/ folder.
Submission Bugs

Fixed a couple of submission bugs where the uploaded file would not be
renamed. Also fixed the URL incorrectly displaying.
Submissions Bug

Fixed a bug where only the first assignment would accept submissions
correctly.
File Download Security & Database

File downloads are now secure, as it checks if you have proper
permissions before starting the file download. The database was also
slightly altered, and I have included a mydb_structure.sql that
contains only the structure of the database.