Add a short README describing the driver

Author: mtvec

README.md

@@ -1,2 +1,15 @@
-# PmodClsArduino
-PmodCLS driver for Arduino
+# PmodCLS driver for Arduino
+
+This is a simple Arduino driver for the Digilent [PmodCLS](http://store.digilentinc.com/pmodcls-character-lcd-with-serial-interface/) LCD module.
+It is meant to be as much as a drop-in replacement for [LiquidCrystal](https://www.arduino.cc/en/Reference/LiquidCrystal) as possible.
+However, the following incompatibilities currently exist:
+ - The autoscroll feature is not present on the PmodCLS.
+   This could be implemented in software but currently is not;
+ - The right-to-left feature is not present either.
+   This is probably impossible to emulate in software;
+ - Defining custom characters *is* supported by the hardware but not implemented yet;
+ - The constructors are completely different.
+
+Since there are different ways to communicate with the PmdCLS (UART, SPI and I²C), this driver does not talk to the hardware directly.
+Instead, it writes a command stream to a user-provided function which is responsible of communicating with the hardware.
+See the [HelloWorld](examples/HelloWorld/HelloWorld.ino) example for a simple way to communicate over UART.