boards: cavs15: add tools/README.md comparing loggers

The correct information is from Andy Ross and the mistakes from me.

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

boards/xtensa/intel_adsp_cavs15/tools/README.md

@@ -0,0 +1,22 @@
+For various legacy reasons this directory has two similar log tools:
+logtool.py and adsplog.py
+
+Both may be used in automation so merging them would require some
+coordination.
+
+They both read from the same data from the exact same shared memory
+yet they have significant differences:
+
+- logtool.py reads /sys/kernel/debug/sof/etrace which requires the
+  kernel driver to be loaded.
+- adsplog.py finds the memory address by scanning
+  /sys/bus/pci/devices/; this does not require a driver.
+
+- logtool.py supports reading from a special QEMU location.
+
+- logtool.py performs a raw dump of the memory and exits immediately.
+- adsplog.py parses the data, understands the ring buffer and reads
+  continuously. Its output is much more human-readable.
+
+- adsplog.py has technical details explained in a comment at the top.
+