Exclude irrelevant files from npm releases

[callahad]

Before we publish a first release, it probably makes sense to exclude extraneous files, like the whole process/ directory, from the npm artifacts.

With this change, we'll publish the following to npm:

package/
├── CHANGELOG.md
├── LICENSE
├── README.md
├── css/
│  ├── quotes.css
│  ├── remedy.css
│  └── reminders.css
└── package.json

[callahad]

I went back and forth on the CONTRIBUTING file... landed on not including it since actually contributing would likely need a full checkout of the repo anyway, rather than poking around in node_modules/, and the README does explicitly say that we welcome contributions.

We can add it later if we want :)

We may also want to eventually provide pre-minified builds with source maps in our npm releases, but this gets us started for now.

[mirisuzanne]

makes sense to me, thanks!