Add .npmignore

[jhermsmeier]

Ignore unnecessary files to slim published module, so that it doesn't contain build artefacts etc.

[ericelliott]

Does this exclude package.json? Does that work?

[jhermsmeier]

The .npmignore only tells npm to ignore the files / directories listed in it when running
npm publish, so that they don't end up being in the tarball, much like how .gitignore works.
It does not exclude the package.json (see https://docs.npmjs.com/misc/developers#keeping-files-out-of-your-package for details).

This will just keep the other build files and the sources out, as that's not loaded or used when installed via npm.