Basic example code usage in readme

Hi I am struggling to get this package working. Seems to me that I have connection to mongo db setup properly, because I could list my collections names from calling `db._getCollectionNames`

but I am not able to figure out how to get collection, so I can perform update on collection:

```
exports.up = function (db) {
// how to use db to get to collection?
// this won't work
db.collection('Profile').update({},
{ $set: { "termsOfServiceUpdated": true } },
{
upsert: false,
multi: true
});
};
```

thank you very much

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