Developers documentation added

Since issue #44 brought some major changes to the block, here is documentation targeted to further Accessibility Block developers to be able to get into the code more easily.

It contains:
* Basic working principle
* Setup advice for block appearance throughout entire Moodle site
* block_accessibility.php program flow diagram
* userstyles.php program flow diagram
* changesize.php and changecolour.php program flow diagrams
* database.php script explanation and table scheme diagram