rs: code friendly keymap for preonic, iris and my preonic clone (#4303)

I worked on those keymap to simplify the use of 60% keyboards for
coders. Instead of trying to mimic planck, this keymap remove
raise/lower layer complexity and keep some important sign keys for coder
in the upper right side pretty much the same way as they are on a full
keyboard.

A karabiner configuration is also provided to mimic most of the keymap
features on the macbook internal keyboard for when you are forced to use
it.

keyboards/handwired/rs60/keymaps/default/readme.md

@@ -0,0 +1 @@
+# The default Preonic layout - largely based on the Planck's