Added reference to Windows and macOS icon formats

README.md

@@ -4,10 +4,21 @@
 
 A collection of graphical Icons used to denote the internal framework of the Interoperable Master Format, as used in the Essential Training - IMF course.
 
+Each filetype icon is available in four file formats:
+ * original PNG format by @trenchdigital: 512×512 pixels, RGBA (32 bits/pixel)
+ * Windows icon file format (`.ico` extension): 512×512 down to 16×16 pixels, RGBA (32bpp), 256-color and 16-color
+ * macOS icon file format (`.icns` extension):  512×512 down to 16×16 pixels, RGBA (32bpp), 256-color and 16-color
+ * One unique icon library (`.icl` extension) collecting all of the filetype icons (in all of their variants) in one file, usable by any OSs; this latter file is Gzipped to reduce storage footprint
+
+Icons originally designed by [Jack Watts](https://github.com/jack-watts).
+Conversions into OS icons by [Walter Arrighetti](http://github.com/walter-arrighetti).
+
+Sizes 512×512 down to 16×16 pixels, at 32bits/pixel, 256-colors and 16-colors (EGA palette), with nonlinear scaling of graphic details (“opticals”).
+
 [Go to training course.](https://training.trenchdigital.net/imf)
 
 ## Meta
 
 Distributed under the BSD 2-Clause License license. See ``LICENSE`` for more information.
 
-[Github](https://github.com/trenchdigital) | [Website](https://trenchdigital.net) | [Contact](https://trenchdigital.net#contact)
+[Github](https://github.com/trenchdigital) | [Website](https://trenchdigital.net) | [Contact](https://trenchdigital.net#contact)