doc: update release notes

Add more notes and changes to doc.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

Author: nashif

release-notes.md

@@ -1,5 +1,27 @@
 # Notes / Changes in various releases
 
+## Zephyr SDK 0.18.0-alpha4
+
+- general:
+
+  * Added Clang/LLVM 19.1.1 toolchain with Arm and RISC-V architecture support.
+  * Update to GCC 14.2
+  * Picolibc is configured as the main and default C library.
+  * Dropped support for newlib and newlib-nano
+  * Dropped support NIOS II architecture
+  * Introduced Clang/LLVM toolchain distribution bundle
+    (`zephyr-sdk-VER_HOST_llvm`).
+  * Renamed existing "full" GNU toolchain distribution bundle to
+    `zephyr-sdk-VER_HOST_gnu`.
+  * Windows executables now link against UCRT, which supports Unicode.
+
+- cmake:
+
+  * Restructured toolchain CMake script to support GNU and LLVM toolchains.
+    This requires Zephyr 4.2.0 or above (or the latest `collab-sdk-dev`).
+  * Zephyr SDK GNU toolchain now declares `TOOLCHAIN_HAS_GLIBCXX`.
+  * Zephyr SDK LLVM toolchain now declares `TOOLCHAIN_HAS_LIBCXX`.
+
 ## Zephyr SDK 0.18.0-alpha2
 
 - general: