Added README

README.md

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+# Arduino-based PS2 -> Macintosh Plus keyboard adapter
+
+This sketch allows to use a PS/2 keyboard with the Macintosh Plus. Mapping between PS/2 keys and Mac keys is stored in the [keymap.ino](keymap.ino) file. Sketch uses a modified version of the [PS2Keyboard Library](http://www.pjrc.com/teensy/td_libs_PS2Keyboard.html) --- all the logic except `getScanCode()` method have been stripped.
+
+## Connecting to Macintosh Plus
+
+Get a phone cord with RJ10 plug and connect pin as follows:
+
+1. +5V,
+2. pin 5,
+3. pin 6,
+4. GND.
+
+The order of RJ10 pins is the same as in the computer connector:
+
+![RJ10 pinout](doc/mac-plus-keyboard-pinout.png)
+
+## Connecting to PS/2 keyboard
+
+For the Mini-DIN 6 plug, pins should be connected as follows:
+
+1. pin 2,
+2. -,
+3. GND,
+4. +5V,
+5. pin 6,
+6. -