Create itm.md

docs/reference/contributing/target/itm.md

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+### Instrumented Trace Macrocell
+
+For targets with Arm CoreSight (for example, Cortex-M3 and Cortex-M4), the Instrumented Trace Macrocell provides a lightweight, nonintrusive way to collect debug trace output. 
+
+#### Assumptions
+
+- The target supports Arm CoreSight.
+- The target has SWO connected either to a compatible interface chip or exposed as a debug pin.
+
+##### Defined behavior
+
+- Targets must implement the function `void itm_init(void)` and add `ITM` to the `device_has` section in `target.json`.
+- When `void itm_init(void)` is called, the debug clock for the ITM must be initialized and the SWO pin configured for debug output.
+- `void itm_init(void)` only has to modify the clock prescaler in the generic register `TPI->ACPR`. The helper function `mbed_itm_init` initializes the generic ITM registers. 
+- `void itm_init(void)` is only called once during startup and doesn't have to be protected for multiple calls.
+
+##### Undefined behavior
+
+- The debug clock frequency is left undefined because the most optimal frequency varies from target to target. It is up to each target's owner to choose a frequency that doesn't interfere with normal operation and that the owner's preferred debug monitor supports. 
+
+#### Testing
+
+You can use the `SerialWireOutput` to send `stdout` to the SWO stimulus port on the ITM by including this override function:
+
+```
+#include "SerialWireOutput.h"
+
+FileHandle* mbed::mbed_override_console(int fd) {
+    static SerialWireOutput swo;
+    return &swo;
+}
+```
+
+You can place the function in any C++ compilation unit, including `main.cpp`.