Update inputprocessing.md

Hi,

I haven't got round to switching from the 'rock solid' Pi image to the local emonCMS installation so I can't contribute anything to the emonCMS codebase, but I've had a go at improving the documentation for input processing. I figure most people won't want to have to read the code to understand what some of the less popular ones do..

In particular, I added a note to the effect that you can use kwh_to_kwhd to log the daily increase in value of any accumulator feed, since the accumulator process returns the new total value. (Ideally that process would have a more generic name, like "Accumulating input to daily feed" - hindsight I guess!)

cheers,

Dave