Add ChangeLog entry for SE-0313, which is now implemented.

Author: DougGregor

CHANGELOG.md

@@ -29,6 +29,31 @@ CHANGELOG
 Swift 5.5
 ---------
 
+* [SE-0313][]:
+
+  Parameters of actor type can be declared as `isolated`, which means that they
+  represent the actor on which that code will be executed. `isolated` parameters
+  extend the actor-isolated semantics of the `self` parameter of actor methods
+  to arbitrary parameters. For example:
+
+  ```swift
+  actor MyActor {
+    func f() { }
+  }
+
+  func g(actor: isolated MyActor) {
+    actor.f()   // okay, this code is always executing on "actor"
+  }
+
+  func h(actor: MyActor) async {
+    g(actor: actor)        // error, call must be asynchronous
+    await g(actor: actor)  // okay, hops to "actor" before calling g
+  }
+  ```
+
+  The `self` parameter of actor methods are implicitly `isolated`. The
+  `nonisolated` keyword makes the `self` parameter no longer `isolated`.
+
 * [SR-14731][]:
 
   The compiler now correctly rejects the application of generic arguments to the