Mention PBox2D -> Box2DProcessing in README.md

[eggsyntax]

Hi Daniel! Thanks so much for your work on Box2D-for-Processing, and for making the Nature of Code book freely available (I think I actually bought a copy when it came out, but it's been a long time...). I just tried to set up a project based on the book and tripped over the PBox2D class being unavailable, and found in the Processing forum that it'd been replaced by Box2DProcessing.

PR just makes brief mention of that in the README. It may be that there are other changes that have to be taken into account; if I stumble across more I'll try to create PRs for them as I have time.

Thanks again!

[eggsyntax]

(alternately if Box2D is long-outdated and there's some other physics engine lib that everyone uses these days, I'd be grateful to know that :) )

[eggsyntax]

Update: everything else seems to be working well, although it might be worth naming some of the necessary imports -- I've been finding them as I go along by searching this repo. The ones I've needed so far are

import shiffman.box2d.*;
import org.jbox2d.dynamics.*;
import org.jbox2d.common.Vec2;
import org.jbox2d.collision.shapes.*;