Add a pointer to OpenPrinting CUPS and clarify that Apple CUPS is what is shipped with macOS and iOS.
-README - CUPS v2.3.5 - 2021-04-26
-=================================
+README - Apple CUPS v2.3.5 - 2021-09-13
+=======================================
+
+> Note: Apple CUPS is the version of CUPS that is shipped with macOS and iOS.
+> For the current version of CUPS that is used on other operating systems, see
+> <https://openprinting.github.io/cups> for details.
+
INTRODUCTION
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