bpo-38439: Add 256px IDLE icon to the .ico, drop gifs from it (GH-19648)

diff --git a/Lib/idlelib/Icons/README.txt b/Lib/idlelib/Icons/README.txt
index 8b47162..d91c4d5 100644
--- a/Lib/idlelib/Icons/README.txt
+++ b/Lib/idlelib/Icons/README.txt
@@ -7,3 +7,7 @@
 
 Various different formats and sizes are available at this GitHub Pull Request:
 https://github.com/python/cpython/pull/17473
+
+The idle.ico file was created with ImageMagick:
+
+    $ convert idle_16.png idle_32.png idle_48.png idle_256.png idle.ico