specify files to pack to avoid oclif warning
specify files to pack to avoid oclif warning

this attribute is now required in plugins otherwise a warning is displayed (though notably, the plugins still work fine with the warning)

See my blog post for the reason why this is now required: https://medium.com/@jdxcode/for-the-love-of-god-dont-use-npmignore-f93c08909d8d

for other plugins, if you run `npm pack; tar -xvzf *.tgz; rm -rf package *.tgz` that will print the files it's currently packing

heroku/heroku-pg-extras