🎨 Add brush source

assets/README.md

@@ -1,7 +1,20 @@
-# Notes for `claws.blend`
+# `claws.blend`
 
 This is the source file for `resources/img/claws.png`. To generate the image, the animation in `claws.blend` is rendered to 256 PNG files and then combined using this command:
 
 ```bash
 convert *.png -average claws.png
-```
+```
+
+# `brush.blend`
+
+This is the source file for `resources/img/brush.png`. To generate the image, the four dynamic paint image sequences in `brush.blend` are baked to 256 PNG files each. They are then combined using these commands:
+
+```bash
+convert brush1/*.png -average brush1.png
+convert brush2/*.png -average brush2.png
+convert brush3/*.png -average brush3.png
+convert brush4/*.png -average brush4.png
+
+convert brush1.png brush2.png brush3.png brush4.png -channel RGBA -combine brush.png
+```