Readme - add STDIO section

Author: 0xc0170

readme.md

@@ -9,3 +9,14 @@ for more details).
 Because of this, you'll always want to make `mbed-drivers` a dependency of your mbed OS application. Check
 [our "Getting started" guide](https://docs.mbed.com/docs/getting-started-mbed-os/) for more details about `mbed-drivers`
 and mbed OS in general.
+
+### STDIO retartgeting
+
+The mbed-drivers defines retargeting of stdin, stdout and stderr to UART. The default baudrate for STDIO UART peripheral is set via ```YOTTA_CFG_MBED_OS_STDIO_DEFAULT_BAUD```. If this yotta config is not defined, the default value is 115200.
+
+To change STDIO serial settings in the runtime, retrieve the Serial STDIO object ```get_stdio_serial()```.
+
+```
+Serial& pc = get_stdio_serial();
+pc.baud(9600);
+```

test/serial_interrupt/main.cpp

@@ -38,6 +38,7 @@ void app_start(int, char*[]) {
     MBED_HOSTTEST_SELECT(echo);
     MBED_HOSTTEST_DESCRIPTION(serial interrupt test);
     MBED_HOSTTEST_START("MBED_14");
+    computer.baud(115200);
     computer.attach(&txCallback, Serial::TxIrq);
     computer.attach(&rxCallback, Serial::RxIrq);
 }