Update changelog for AddPolicy (#25394)

Author: annelo-msft

sdk/core/Azure.Core/CHANGELOG.md

@@ -10,6 +10,7 @@
 ```c#
 options.Diagnostics.LoggedQueryParameters.Remove("api-version");
 ```
+- Added `AddPolicies` method to `RequestContext`.  This allows policies to be added to the pipeline when calling protocol methods.
 
 ### Bugs Fixed