release-notes: Add release notes for 0.15.0-beta2

This commit adds the release notes for the Zephyr SDK 0.15.0-beta2
release.

Signed-off-by: Stephanos Ioannidis <[email protected]>

Author: stephanosio

release-notes.md

@@ -1,6 +1,6 @@
 # Notes / Changes in various releases
 
-## Zephyr SDK 0.15.0-beta1
+## Zephyr SDK 0.15.0-beta2
 
 - general:
   * Added ARCv3 support to the toolchains.