add README

README.md

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+# Super Simple UART config for ATmega
+
+lets you use normal `printf()` in your code.
+
+## Adjustments to make
+the dafault configuration works on an atmega328p with 38400 baud. Any other setup might require one of the following changes.
+
+##### Set baudrate in 'uart.h'
+line 6:
+`#define BAUD 38400`
+
+##### Select which UART to use
+If your controller has more than one UART hardware and you want to use a different than UART0,
+change the register defines in the following way:
+
+from `#define UART_BAUD_REGH UBRR0H` to `#define UART_BAUD_REGH UBRR3H`
+and
+from `#define UART_UCSZ0_BM UCSZ00` to `#define UART_UCSZ0_BM UCSZ30`