add files to package.json

[kentcdodds]

If you look here you'll notice that there are several files included with this package on the registry that don't need to be. This causes npm installs of this module to result in downloading more than necessary.

By adding this files array to package.json you can specifically list the files that are important for people to download for your module. By default, npm will also include README and LICENSE files as well.

[coveralls]

Coverage remained the same at 95.588% when pulling 6248414 on kentcdodds:patch-1 into 5511994 on davidkalosi:master.

[kentcdodds]

Oh, and you can test this locally by running npm pack and opening up the resulting zip file.

[davidkalosi]

Thanks for the PR, will release an updated npm package later this day