接口和结构重载？

I am completely new to go and am planning on building a bittorent client as my first project to learn the language. One aspect is econding and decoding of bittorrent files. An example that I've found creates a struct containing a bufio.Reader, and then uses some interfaces to add additional methods. Can someone explain the mechanism for constructing these interfaces in such a way that overloads the struct? I'm not clear on how the functions in this file are being added to the decoder struct (https://github.com/marksamman/bencode/blob/master/decoder.go):

package bencode

import (
    "bufio"
    "errors"
    "io"
    "strconv"
)

type decoder struct {
    bufio.Reader
}

func (decoder *decoder) readDictionary() (map[string]interface{}, error) {
    dict := make(map[string]interface{})
}

func (decoder *decoder) readString() (string, error) {
    ...
}

func (decoder *decoder) readList() ([]interface{}, error) {
    ...
}

func Decode(reader io.Reader) (map[string]interface{}, error) {
    decoder := decoder{*bufio.NewReader(reader)}
    if firstByte, err := decoder.ReadByte(); err != nil {
        return make(map[string]interface{}), nil
    } else if firstByte != 'd' {
        return nil, errors.New("bencode data must begin with a dictionary")
    }
    return decoder.readDictionary()
}