npm WARN package.json: No repository field
It's just a check as of NPM v1.2.20, they report this as a warning.
However, don't worry, there are sooooooo many packages which still don't have the repository field in their package.json. The field is used for informational purposes.
In the case you're a package author, put the repository in your package.json, like this:
"repository": {
"type": "git",
"url": "git://github.com/username/repository.git"
}