Appends a string to a string field.
If the string field does not exist in the document, it just sets it.

db.collection.insert({_id: "test", message: "Mongo"}); //message is now "Mongo"

db.colection.update({_id: "test"}, {$append:

, false, false}; //message is now "MongoDB"

db.colection.update({_id: "test"}, {$append:

, false, false}; //anotherMessage is now "NoSQL"