Added how and why.
Updated attribution of example source.
Started to work on digital IO control.
Added more DIO functions.
Updated the way RegisterState works.

Now its state includes a tuple of 'set' and 'clear' words. Instead of
accumulating the *desired value* of the register all through, it
accumulates these set and clear words.
The updateRegister function generates an Atom action to apply these set
and clear words.
Updated the Watchdog to use the new register style.
Updated DigitalIO to use new register style.
Optimised action generation for RegisterState updates.
Added reset convenience function to RegisterState.
Merge branch 'dev'
Added link to roadmap.