Add example for writing to target

example/target.lua

@@ -0,0 +1,18 @@
+local vips = require "vips"
+
+if #arg ~= 2 then
+    error("Usage: lua test.lua ~/pics/k2.png .avif > x")
+end
+
+local infilename = arg[1]
+local fmt = arg[2]
+
+local descriptor = {
+    stdin = 0,
+    stdout = 1,
+    stderr = 2,
+}
+
+local image = vips.Image.new_from_file(infilename)
+local target = vips.Target.new_to_descriptor(descriptor.stdout)
+image:write_to_target(target, fmt)