Add release note for CodeBlockSyntax becoming SyntaxParseable.

Author: DougGregor

Release Notes/511.md

@@ -24,6 +24,10 @@
   - Description: A trait for syntax nodes that have an optional code block, such as `FunctionDeclSyntax` and `InitializerDeclSyntax`.
   - Pull Request: https://github.com/apple/swift-syntax/pull/2359
 
+- `CodeBlockSyntax`
+  - Description: `CodeBlockSyntax` is now `SyntaxParseable`, so it can be used with string  interpolations.
+  - Pull Request: https://github.com/apple/swift-syntax/pull/2361
+
 ## API Behavior Changes
 
 ## Deprecations