Improve type class visibility in README.md

[RRRRRRR-777]

The README.md's "Check type class" section uses the IsImage class as an example, but it would be much easier to understand if examples of other file type classes were also included. Please consider improving this.

Below is an example of how I would document it. Please refer to it.
It would also be good to include this in the official documentation.

https://pkg.go.dev/github.com/h2non/filetype

Check type class
```
package main

import (
  "fmt"
  "io/ioutil"

  "github.com/h2non/filetype"
)

func main() {
  buf, _ := ioutil.ReadFile("sample.jpg")

  if filetype.IsImage(buf) {
    fmt.Println("File is an image")
  } else {
    fmt.Println("Not an image")
  }
}
```

**Available File Type Classes**
* IsImage: Checks if the file matches an image format
* IsAudio: Checks if the file matches an audio format
* IsVideo: Checks if the file matches a video format
* IsFont: Checks if the file matches a font format
* IsArchive: Checks if the file matches an archive format
* IsDocument: Checks if the file matches a document format
* IsApplication: Checks if the file matches an application format
* Is: Checks if the file matches the magic number for the specified extension
* IsExtension: Alias for Is()