doc: release notes: add HAL section

Adding a minor section about HAL changes
in the 2.0.0 release notes.

Signed-off-by: Ioannis Glaropoulos <[email protected]>

Author: ioannisg

doc/releases/release-notes-2.0.rst

@@ -462,7 +462,8 @@ Libraries / Subsystems
 HALs
 ****
 
-* TBD
+* HALs are now moved out of the main tree as external modules and reside
+  in their own standalone repositories.
 
 Documentation
 *************