Add README.md to OSI interface

hardware/interface-osi/README.md

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+* OSI Compatible interface
+An interface for OSI computers that is compatible with the OSI 542C keyboard, including sound circuits.  
+
+This interface also permits substitution of a soft-shiftlock function using 
+a regular momentary keyswitch, instead of the hard-to-fine latchine keyswitch.
+An LED is provided to indicate SHIFT LOCK status.  This interface mates with
+the Classic ASCII keyboard, configured with the OSI keycaps.  
+
+**Construction Notes:
+
+1. If a diode per key is installed on the keyboard, then omit diodes D1-D8 and install D18.
+
+1. If the keyboard is not installed with a diode per key, then install D1-D8, and omit D18.
+
+1. The RESET delay is about 3 seconds with a 6.8 uF capacitor at C20.  4.7 uF gives about
+2 seconds, and 3.3 uF gives about a 1 second delay.
+
+1. If you want to use the sound circuit with an older 540A video board, then jumper pin 13
+of IC E4 to pin 15 of the keyboard connector, and set JP1 to "S1".
+
+1. If you use a 540B video board, then set JP1 to "/S1"
+