Updated how to disable Settings screen in docs

[tcoukoulis]

The documentation in docs/usage.md incorrectly instructs developers to add a method calling the add_filter method inside the wp-config.php file before:

require_once(ABSPATH . 'wp-settings.php');

This results in a fatal error, per my comments in #162. I have edited the line in the documentation to instead define a constant as expected at https://github.com/jchristopher/attachments/blob/master/classes/class.attachments.php#L1826.

[jchristopher]

Thank you!