doc: apply pngcrush to Getting Started images.

doc/snippets/README.md

@@ -5,7 +5,11 @@ Snippets that possibly generate output for Magnum documentation
 
 Displayed by the `getting-started` executable. Run the app and take screenshot
 using Spectacle (including decorations, 880x707). Similarly for the gray
-version. The resulting files should be resized to half.
+version. The resulting files should be resized to half. `pngcrush` to them for
+smaller file sizes:
+
+    pngcrush -ow getting-started.png
+    pngcrush -ow getting-started-blue.png
 
 The output printed by the application can be used to update the example output
 in `doc/getting-started.dox`.