Update README.md

Author: Bitl

README.md

@@ -28,6 +28,7 @@ This base is specific to multiplayer mod projects.
 - Multiplayer NPC support with the BDSBASE_NPC preprocessor definition.
 - Various optional preprocessor definitions for changing TF2's behavior.
 - Menu music in map backgrounds with adjustable volume and mix volume.
+- previous2021 beta detection, allowing players to be informed if they're launching from the wrong SDK 2013 version. This can be disabled with -ignoreprevious2021check.
 
 ## Options/Preprocessor Definitions
 These options are meant to be added to the VPC files (client and server) of the mod you wish to modify.