readme: add external types import to example

[oleg-jukovec]

A user must import packages of external msgpack types: datetime, decimal, uuid to be able to parse them from a response:

import (
     _ "github.com/tarantool/go-tarantool/v2/datetime"
     _ "github.com/tarantool/go-tarantool/v2/decimal"
     _ "github.com/tarantool/go-tarantool/v2/uuid"
)

We need to include this lines to the root example in README.md and comment it to make it clear.