expand readme a bit

Author: jcupitt

README.md

@@ -6,7 +6,15 @@ Status](https://build.snapcraft.io/badge/jcupitt/nip2.svg)](https://build.snapcr
 nip2 is a GUI for the [libvips image processing 
 library](https://libvips.github.io/libvips). It's a little like a spreadsheet:
 you create a set of formula connecting your objects together, and on a change
-nip2 recalculates.
+nip2 will recalculate. This makes it convenient for developing image processing
+systems since you can watch pixels change as you adjust your equations.
+
+Because nip2 uses libvips as the image processing engine it can handle very
+large images and only needs a little memory. It scales to fairly complex
+workflows: I've used it to develop systems with more than 10,000 cells,
+analyzing images of many tens of gigabytes. It has a batch mode, so you
+can run any image processing system you develop from the command-line and
+without a GUI.
 
 [![Screenshot](screenshot.png)](screenshot.png)