Add more term definitions

The dictionary for definition of commonly used terms is getting rather large. I propose that it is moved into a separate, standalone documentation file. A message should then be placed at the top of the `README.md`, stating that readers should familiarise themselves with all the defined terms before continuing reading (to prevent confusion).

In addition, the terms "command", "event", "interaction", "event listener" and "task" should have definitions added to that new documentation file, alongside the moved definitions from the `README.md`