Update changelog w/note about public namespace change for 2.6 release (#719)
Update changelog w/note about public namespace change for 2.6 release (#719)

* update change log for 2.6 release

* add dual serialization comment

AzureAD/microsoft-authentication-library-for-dotnet,AzureAD/microsoft-authentication-library-for-dotnet,AzureAD/microsoft-authentication-library-for-dotnet,AzureAD/microsoft-authentication-library-for-dotnet