add entry to feature-switches doc

docs/workflow/trimming/feature-switches.md

@@ -13,6 +13,7 @@ configurations but their defaults might vary as any SDK can set the defaults dif
 | EnableUnsafeBinaryFormatterSerialization | System.Runtime.Serialization.EnableUnsafeBinaryFormatterSerialization | BinaryFormatter serialization support is trimmed when set to false |
 | EventSourceSupport | System.Diagnostics.Tracing.EventSource.IsSupported | Any EventSource related code or logic is trimmed when set to false |
 | InvariantGlobalization | System.Globalization.Invariant | All globalization specific code and data is trimmed when set to true |
+| PredefinedCulturesOnly | System.Globalization.PredefinedCulturesOnly |  Don't allow creating a made-up cultures when the platform does not carry data for it |
 | UseSystemResourceKeys | System.Resources.UseSystemResourceKeys |  Any localizable resources for system assemblies is trimmed when set to true |
 | HttpActivityPropagationSupport | System.Net.Http.EnableActivityPropagation | Any dependency related to diagnostics support for System.Net.Http is trimmed when set to false |
 | UseNativeHttpHandler | System.Net.Http.UseNativeHttpHandler | HttpClient uses by default platform native implementation of HttpMessageHandler if set to true. |