Add local setup instructions to README

[exogen]

I couldn't figure out how to actually run this (locally, so I could make changes).

When I open index.html, I just get ReferenceError: Elm is not defined. That was after installing Elm and doing elm-package install in src which was just a guess on my part. That didn't seem to fix anything.

Instructions for setting this up would be nice!

[evancz]

I'd switch to the gh-pages branch. That has all the assets set up already. From there you can recompile things however you want. I'll add a note about this though!

[evancz]

Note added!