Add cards and save cards example

[staticdev]

Thanks for the library, I was able to see my contacts. Since, I am new to golang, and just wanted to see if it is possible that you provide a way to add a new contact and save all contacts in documentation.

[emersion]

There's already an example for mutating vcards: https://godocs.io/github.com/emersion/go-vcard#example-NewEncoder