Document the change policy for FileExtensionContentTypeProvider #2454 (#8213)

src/Middleware/StaticFiles/src/FileExtensionContentTypeProvider.cs

@@ -11,6 +11,10 @@ namespace Microsoft.AspNetCore.StaticFiles
     /// </summary>
     public class FileExtensionContentTypeProvider : IContentTypeProvider
     {
+        // Notes:
+        // - This table was initially copied from IIS and has many legacy entries we will maintain for backwards compatibility.
+        // - We only plan to add new entries where we expect them to be applicable to a majority of developers such as being
+        // used in the project templates.
         #region Extension mapping table
         /// <summary>
         /// Creates a new provider with a set of default mappings.