📝 Add note regarding status of the DT-provided Sequelize types (#35173)

types/sequelize/index.d.ts

@@ -20,6 +20,11 @@
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
 // TypeScript Version: 2.8
 
+// ***************************** IMPORTANT NOTE *****************************
+// These types are for the 4.x branch of Sequelize. As of Sequelize 5.0,
+// types are packaged directly within Sequelize itself. Please target the
+// Sequelize-provided types for any changes related to versions >= 5.0.
+
 // Based on original work by: samuelneff <https://github.com/samuelneff/sequelize-auto-ts/blob/master/lib/sequelize.d.ts>
 
 /// <reference types="validator" />