[Docs] Need to document that Bitmap scaling requires BaseSize to be set

[kazo0]

Based on the current README in the Resizetizer.NT repo, we need to document something similar to https://github.com/Redth/ResizetizerNT#bitmaps

Specifically surrounding the fact that PNGs that are added as UnoImage will not get scaled and resized unless a BaseSize is defined on the UnoImage item