Added a README file.

README.md

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+gint project
+============
+
+gint (pronounce guin) is a low-level library for fx-9860G calculators. It
+provides the tools needed to develop programs under Linux using the gcc
+toolchain (sh3eb-elf).
+
+By the way, gint is free software ; you may use it for any purpose, share it,
+modify it and share you changes. No credit of any kind is needed, though
+appreciated.
+
+
+
+Interrupt handler
+-----------------
+
+The interrupt handler is the lowest-level part of the library. It directly
+accesses the peripheral modules and performs keyboard analyzes, swaps screen buffers, etc.
+
+gint does not allow user programs to use their own handlers. However, it is
+possible to map interrupt-driven events to user callbacks using the public API
+(which is not possible with the system's interrupt handler). This may be
+particularly useful for timers and RTC (the 16 Hz interrupt can be used as a
+basis for a physical engine).
+
+
+
+Public Interface
+----------------
+
+gint's API provides access to keyboard, timers, clock and more. It does some
+powerful drawing and offers reliable multi-getkey, a gray engine, facilitates
+register access and implements a few standard functions.