update - add values using $set

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examples/mongodb/update_add_values.txt

> db.users.update({ name : 'Zorg' }, { $set : { child: 'Jane' } })

> db.users.find()
{ "_id" : ObjectId("526b8fdba444e81f1ca06ba2"), "name" : "Foo" }
{
    "_id" : ObjectId("526b9048a444e81f1ca06ba3"),
    "name" : "Zorg",
    "child" : "Jane"
}
{ "_id" : ObjectId("526ba048a444e81f1ca06ba4"), "name" : "Moo" }