Add note about vista_data.uf2

software/Readme.md

@@ -53,6 +53,8 @@ Plays the Touhou music video at 720p 30FPS (no sound). Because the video is larg
 
 Full-resolution VGA RGB565 image viewer. As each raw image is over 600 kB, only a small slice of the image can reside in memory at once, so images are streamed continuously from flash at 40 MB/s. The TMDS encode uses around 95% of both cores, which is why we need a raw image format. The remaining 5% of core 1 handles the horizontal DMA interrupts to keep the DVI signalling going, and core 0 handles the streaming of the image from flash. This is not very practical, but it was a fun project.
 
+*NOTE: you must first copy the [image data UF2](assets/vista_data.uf2) before running this app*
+
 If you define the symbol `TMDS_FULLRES_NO_DC_BALANCE` then you can remove the DC balance feedback from the TMDS encode, which *may* give you enough time to do something more interesting with full-resolution RGB graphics, provided your DVI signals are DC-coupled, and your TV is feeling lenient. This is absolutely forbidden by the standard, but don't worry, I won't tell anyone if you do this.
 
 ### Terminal