Added KBD66 (#2292)

* Created default layout for KBD66.

* Added KBD66 ISO layout.

keyboards/kbd66/keymaps/default/readme.md

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+# The default keymap for KBD66
+
+![KBD66 Layout Image](https://i.imgur.com/QNpHnMe.jpg)
+
+This is a fairly universal layout that supports standard ANSI, split backspace,
+split left shift, and split right shift.  
+Cycle backlight: Fn + .  
+Backlight breathing: Fn + ,  
+Volume Up/Down: Fn + [ and Fn + ]  
+Mute: Fn + \  
+Sleep: Fn + /  
+Bootloader Mode: Fn + CapsLock