Update the readme and the demo page text

[rocketnia]

The readme and the demo page could be updated in a few ways:

• Give more up-to-date instructions for how to install and run the project. Let's formulate some specific advice for how to set up Node.js applications and front-end web libraries that run Arc code using Rainbow.js.
• Reevaluate how the performance compares with Java Rainbow. We should at least tone down the old excitement we had that was based on a version of Rainbow.js that didn't pass the unit tests, but we might have a more interesting perspective to show if the benchmarks can run (See if we can run the Rainbow benchmarks on Rainbow.js now #5).
• Most features that don't yet exist are possible to "implement" in the same way we do already for System_out, System_getenvAsync0, and so on, where different wrappers like src/node/rainbow-node.js and src/web/index-first.js implement these variables in ways that make the most sense for their platforms. Threads are absolutely possible to implement in terms of continuation-passing style, too, which was something I learned in part thanks to doing this port. While I didn't start porting this project with that extent of parity in mind, it is possible, and let's make sure the readme reflects that potential.

[rocketnia]

As of 339668a, we've updated the readme.

Next up is to update the demo page text.

Edited to add: Actually, we should revise the readme again to document the command rainbow-js-arc init-arc <path>. We describe rainbow-js-arc run-compat [args...], but we neglect to mention that it has to be run from within the kind of Arc host directory rainbow-js-arc init-arc <path> prepares.

[rocketnia]

As of be31738, we've updated the readme and the demo page text.