[OnlineExperimentation] Update CHANGELOG.md with release date. (Azure#50535)

* Update CHANGELOG.md with release date.

* Add --prerelease flag to installation command in README.md.

Author: smichtch

sdk/onlineexperimentation/Azure.Analytics.OnlineExperimentation/CHANGELOG.md

@@ -1,11 +1,7 @@
 # Release History
 
-## 1.0.0-beta.1 (Unreleased)
+## 1.0.0-beta.1 (2025-06-10)
 
 ### Features Added
 
-### Breaking Changes
-
-### Bugs Fixed
-
-### Other Changes
+- Initial release of Azure Online Experimentation client library for .NET, supports service version `2025-05-31-preview`.

sdk/onlineexperimentation/Azure.Analytics.OnlineExperimentation/README.md

@@ -11,7 +11,7 @@ Azure Online Experimentation is a managed service that enables developers to cre
 Install the Azure Online Experimentation client library for .NET with [NuGet](https://www.nuget.org/):
 
 ```dotnetcli
-dotnet add package Azure.Analytics.OnlineExperimentation
+dotnet add package Azure.Analytics.OnlineExperimentation --prerelease
 ```
 
 ### Prerequisites