Updated FAQ (markdown)

FAQ.md

@@ -32,26 +32,10 @@ See [the discussion #137](https://github.com/hajimehoshi/ebiten/issues/137) on o
 
 ## How to take a screenshot?
 
-As a `ebiten.Image` is just a standard `image.Image`, you can pass it to, e.g. `png.Encode`.
+Specify the key to environment variable `EBITEN_SCREENSHOT_KEY` and you can take a screenshot by pressing the specified key.
 
-```go
-func update(screen *ebiten.Image) error {
-        // update logic
-
-        // render to the screen
-
-        if needsToTakeScreenshot {
-                f, err := os.Create("screen.png")
-                defer f.Close()
-                if err != nil {
-                        return err
-                }
-                if err := png.Encode(f, screen); err != nil {
-                        return err
-                }
-        }
-        return nil
-}
+```sh
+EBITEN_SCREENSHOT_KEY=escape go run examples/flappy.go
 ```
 
 ## Ebiten's screen is scaled (and blurred) on high-DPI screen. Can I disable this scaling? In other words, can I have a high-DPI mode?