doc: Add renode command on collecting instruction count

Change-Id: Ie4aa185e66b582706afaa2895efccf3f9f5475ff

RenodeUsageNotes.md

diff --git a/RenodeUsageNotes.md b/RenodeUsageNotes.md
index 8ff9d7a..f9f98c5 100644
--- a/RenodeUsageNotes.md
+++ b/RenodeUsageNotes.md
@@ -85,3 +85,4 @@
   out in Renode.
 * `cpu SaveAllOpcodesCounters @path/to/file`: Dump counter data to a file.
 * `cpu ResetOpcodesCounters`: Reset all counters to 0
+* `cpu ExecutedInstructions`: Report number of instructions executed since the cpu reset