release notes: Add kernel notes

Add notes about new features in the kernel.

Signed-off-by: Anas Nashif <[email protected]>

Author: nashif

doc/releases/release-notes-3.0.rst

@@ -220,6 +220,13 @@ New APIs in this release
 Kernel
 ******
 
+  * Added support for event objects.  Threads may wait on an event object such
+    that any events posted to that event object may wake a waiting thread if the
+    posting satisfies the waiting threads' event conditions.
+  * Extended CPU runtime stats to track current, total, peak and average usage
+    (as bounded by the scheduling of the idle thread).  This permits a developer
+    to obtain more system information if desired to tune the system.
+  * Added "thread_usage" API for thread runtime cycle monitoring.
   * Fixed timeout issues when SYSTEM_CLOCK_SLOPPY_IDLE is configured.
 
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