Add USART documentation.

docs/usart.md

@@ -1 +1,33 @@
 # Usart
+
+## Initializing USART
+
+USART can be initialized with `usart_init()` function after setting a valid
+`struct usart_t` instance.
+
+Code example:
+
+```c
+struct usart_t usart;
+enum usart_result result;
+
+usart.stop_bits = 1;
+usart.baud_rate = 9600;
+usart.data_bits = 8;
+usart.parity = usart_parity_disabled;
+usart.direction = usart_direction_transmit;
+usart.mode = usart_mode_asynchronous_normal,
+
+result = usart_init(&usart);
+```
+
+## Sending Data Over USART
+
+After initializing USART, data can be sent with `usart_transmit` function.
+
+Code example:
+
+```c
+uint8_t data[] = "Hello, world!\r\n";
+result = usart_transmit(&usart, data, sizeof data);
+```