Document a shrinkwrap corner case

Hopefully the documentation on its own is explanation enough.

To trigger that case, use those dependencies in a package.json
```
{
  "name": "bar",
  "version": "0.0.0",
  "dependencies": {
    "fstream": "~0.1.25"
  },
  "devDependencies": {
    "mkdirp": "~0.3.5"
  }
}
```

and then run inside the package:

```
npm install; npm shrinkwrap; rm -r node_modules; npm install --production; npm ls
```

It leaves one with 

```
bar@0.0.0 /home/jzellner/bar
└─┬ fstream@0.1.25
  ├── graceful-fs@2.0.1
  ├── inherits@2.0.1
  ├── UNMET DEPENDENCY mkdirp 0.3
  └── rimraf@2.2.5
```

I don't think this is a bug, but I find it worth mentioning, after spending quite some time tracking it down in a  project with lots of dependencies.