samples: document ways of using prefix

samples/saves/_save_filenames.rb

@@ -0,0 +1,24 @@
+require 'squib'
+
+# This demonstrates the many ways you can save file to save_png and the like
+Squib::Deck.new(width: 50, height: 50, cards: 2) do
+  background color: :white
+  text str: (0..16).to_a, font: 'Arial Bold 12'
+
+  # three digits, e.g. save_three_digits_000.png
+  save_png prefix: 'save_three_digits_', count_format: '%03d'
+
+  # foo_0.png
+  # bar_1.png
+  save_png prefix: ['foo_', 'bar_'], count_format: '%01d'
+
+  # foo.png
+  # bar.png
+  save_png prefix: ['foo', 'bar'], count_format: ''
+
+  # thief.png
+  # thug.png
+  data = csv data: "filename\nthief\nthug"
+  save_png prefix: data.filename, count_format: ''
+
+end