Build process with gulp and and main-bower-files

Hi metafizzy,

It's maybe worth mentioning in the docs that if you are using the `main-bower-files` gulp plugin to add this to your `bower.json`:

```
"overrides": {
"packery": {
"main": "dist/packery.pkgd.js",
"dependencies": {}
}
}
```

Took me quite a while to figure that out.

Review the project's documentation for the build process and determine where guidance for main-bower-files belongs. Document the provided bower.json override for Packery, then check that the example is clear and consistent with the existing documentation.

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