[AVR] Add a release note about the AVR backend becoming an official backend
AVR has been enabled by default since
c480c584a0b7de675dddb2616122fc218cd72c0e, the tests have been stable for
a couple days now, revert extremely unlikely.
diff --git a/llvm/docs/ReleaseNotes.rst b/llvm/docs/ReleaseNotes.rst
index 5b24659..bbfcc60 100644
--- a/llvm/docs/ReleaseNotes.rst
+++ b/llvm/docs/ReleaseNotes.rst
@@ -100,7 +100,9 @@
Changes to the AVR Target
-----------------------------
-During this release ...
+* Moved from an experimental backend to an official backend. AVR support is now
+ included by default in all LLVM builds and releases and is available under
+ the "avr-unknown-unknown" target triple.
Changes to the WebAssembly Target
---------------------------------