build directory ignored by npm

Hi there,

maybe I am doing something the wrong way here but I created a new module by following the instructions in the README.

Then I edited the `source/index.js` and `test/index.js` files accordingly. And then hit `npm publish`.

When I installed it in another project by doing `npm install my-module` if I try include my module somewhere like `const x = require('my-module');` it is unable to resolve it.

I see in the `package.json` file for my module it has this `"main": "build/index.js",` but if I go into the `node_modules/my-module` directory of my project I see there is no `build` directory.

Am I missing something that you know of, and is there possibly something unique with my setup or is this common behaviour of NPM? Thanks in advance

Start with the README instructions, package.json, and the generated package contents after npm publish. Reproduce the install in another project and determine why build/index.js is absent; done means the published module includes the build directory and require('my-module') resolves successfully.

Written by the indexing model from the issue text.