[SE-0412] changelog note about strict concurrency for global variables

Author: sophiapoirier

CHANGELOG.md

@@ -3,6 +3,18 @@
 > **Note**\
 > This is in reverse chronological order, so newer entries are added to the top.
 
+## Swift 5.10
+
+* [SE-0412][]:
+
+  Under strict concurrency checking, every global or static variable must be either isolated to a global actor or be both immutable and of `Sendable` type.
+
+  The attribute `nonisolated(unsafe)` can be used to annotate a global variable (or any form of storage) to disable static checking of data isolation, but note that without correct implementation of a synchronization mechanism to achieve data isolation, dynamic run-time analysis from exclusivity enforcement or tools such as Thread Sanitizer could still identify failures.
+
+  ```swift
+  nonisolated(unsafe) var global: String
+  ```
+
 ## Swift 5.9.2
 
 * [SE-0407][]: