Update FS-1079-union-properties-visible.md

follow up on dotnet/fsharp#11394

Author: smoothdeveloper

RFCs/FS-1079-union-properties-visible.md

@@ -5,6 +5,7 @@ The design suggestion [Make .Tag and .Is* discriminated union properties visible
 This RFC covers the detailed proposal.
 
 - [ ] Implementation: [Proposed](https://github.com/dotnet/fsharp/pull/11394)
+- [x] [Discussion](https://github.com/fsharp/fslang-design/discussions/517)
 
 # Summary