Event Monitor documentation

Add documentation about Event Monitor.

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+# About
+
+The Arm(R) Ethos(TM)-U Monitor is a Python application that reads performance
+data from an Ethos-U (sub)system and outputs the captured data to a file on the
+host machine in either binary or JSON format.
+
+# Ethos-U Monitor porting guidelines
+
+## New input source
+
+The Ethos-U Monitor currently supports three input sources: DAPLink, Linux
+shared memory and file. Adding support for additional input sources requires the
+user to modify [inputs.py](inputs.py) and implement the `InputRingBuffer`
+interface. The `InputDAPLink` class can be used for reference.
+
+```python
+class InputMyCustomInputSource(InputRingBuffer):
+    def __init__(self, elfFile):
+        # Call base class constructor
+        super().__init__(elfFile)
+
+    def read(self, address, size):
+        # Read 'size' bytes from 'address' and return a bytearray
+```
+
+## Linux shared memory
+
+The memory input source (please see `ethosu_monitor.py memory --help`) is
+recommended for Linux Driver Stack, where a Linux host communicates with an
+Ethos-U subsystem using shared memory. Please note that the EventRecorder ring
+buffer must be stored in shared memory that can be accessed by both Linux and
+the Ethos-U subsystem.
+
+The `InputMem` class uses the `/dev/mem` device to read physical memory. The
+user must provide a configuration file that describes the memory mapping between
+Linux and the subsystem. Please see [sample.json](sample.json) for reference.
+
+If for example the Cortex(R)-M in the Ethos-U subsystem access 4MB of shared
+memory at address `0x6000'0000`, and Linux access the same memory region at
+physical address `0x8'8000'0000`, then the entry in the configuration file would
+look like this:
+
+```json
+{
+    "memoryMap": [
+        {
+            "host": "0x880000000",
+            "device": "0x60000000",
+            "size": "0x00400000"
+        }
+    ]
+}
+```
+
+# Firmware
+
+The [baremetal](../../applications/baremetal) application demonstrates how to
+periodically poll the Ethos-U PMU device while an inference is running. PMU
+samples are written to a ring buffer in memory using the
+[EventRecorder](https://github.com/ARM-software/CMSIS-View) library. For users
+that develop their own firmware this application and the libraries it depends on
+serves as reference.
+
+The ring buffer holding the EventRecorder data is recommended to be allocated
+large enough to store all samples for an inference. This is to prevent buffer
+overflows and loss of profiling data.
+
+The ring buffer size is configured by `EventRecorderConf.h`. Please refer to
+[targets/corstone-300/CMakeLists.txt](../../targets/corstone-300/CMakeLists.txt)
+and
+[targets/corstone-300/event_recorder/EventRecorderConf.h](../../targets/corstone-300/event_recorder/EventRecorderConf.h)
+for an example how to override the default EventRecorder settings.
+
+For Linux systems that use `/dev/mem` to read profiling data, the EventRecorder
+BSS segment must be stored in shared memory. For a scatter file it would look
+something like this:
+
+```
+SHARED_MEMORY 0x60000000 0x20000000
+{
+    DDR 0x60000000
+    {
+        EventRecorder.o (+ZI)
+    }
+}
+```
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