Update mediator.mdx

Author: aritchie

src/content/docs/release-notes/mediator.mdx

@@ -8,9 +8,27 @@ import RN from '/src/components/ReleaseNote.astro';
 ## v4
 
 ### v4.10 - TBD
+<RN type="feature">New contract key generator - sample below</RN>
 <RN type="feature">Re-releasing standalone Sentry library due to AOT not being supported on MAUI via Sentry.Diagnostics</RN>
 <RN type="enhancement">New RuntimeEventRegistration service that you can add event handlers to at runtime dynamically</RN>
 
+New Contract Key Generator
+* Type must be partial
+
+```csharp
+[Shiny.Mediator.ContractKeyAttribute("MyClass_{MyProperty}_{Date:yyyyMMdd}")]
+public partial class MyClass 
+{
+  public string? MyProperty { get; set;}
+  public string? UnusedProperty { get; set;}
+  public DateTime? Date { get; set;}
+}
+
+// UnusedProperty will not be included in the contract key
+// If a property is null, it is replaced with an empty string
+// Date will be formatted as yyyyMMdd if not null
+```
+
 ### 4.9.1
 <RN type="fix" repo="mediator" githubNumber="40">Fix storage service deadlock</RN>
 <RN type="fix">Fix another release semaphore state in storage service indexing</RN>