Added PNG export explanation

README.md

@@ -17,4 +17,8 @@ The quality of the output SDF can be previewed by hovering the cursor over the l
 |Setting|Description|
 |---|---|
 |Radius|The radius of the signed distance field. This can be 1 in most cases, but it can be increased if the SDF will be used for things like outline rendering.|
-|Threshold|The input image opacity threshold that is considered opaque. Any input pixel with alpha over this threshold is considered opaque.|
+|Threshold|The input image opacity threshold that is considered opaque. Any input pixel with alpha over this threshold is considered opaque.|
+
+## PNG output
+
+The tool uses [UPNG.js](https://github.com/photopea/UPNG.js/) to produce output .PNG images, since not all browsers can produce .PNG images with colorized transparent pixels natively. In many cases, transparent pixels will become black, losing their color. This causes problems with mipmapping, since the black color blends into the neighboring pixels when downscaling to lower mipmap levels. This tool sets the color of transparent pixels to the color of the nearest opaque pixel and uses UPNG.js generate the output image, preventing the issue.