PicoGraphics: add link

Author: helgibbons

micropython/modules/picographics/README.md

@@ -579,4 +579,4 @@ The arguments for `decode` are as follows:
 1. Decode X - where to place the decoded JPEG on screen
 2. Decode Y
 3. Flags - one of `JPEG_SCALE_FULL`, `JPEG_SCALE_HALF`, `JPEG_SCALE_QUARTER` or `JPEG_SCALE_EIGHTH`
-4. If you want to turn off dither altogether, try `dither=False`. This is useful if you want to pre-dither your images or for artsy posterization effects.
+4. If you want to turn off dither altogether, try `dither=False`. This is useful if you want to [pre-dither your images](https://ditherit.com/) or for artsy posterization effects.