README content is incorrect

[jbphet]

The README file looks like it is the standard fill-in-the-blank thing for a PhET sim, but this repo isn't for a sim. We should probably update the README so that it summarizes what has been done and what we're planning to do in this repo. Among other things, this would help me to decide if I should use this repo or create a new one for the upcoming work that I've been assigned to do for haptics.

I'm going to assign this to @jessegreenberg, since he has done most of the functional commits. @jessegreenberg - If you're uncomfortable with this, assign it back to me and I'll work with the design group to decide what we should have in here.