Fix release notes links (#3411)

Author: martinrrm

docs/release-notes/Index.md

@@ -11,6 +11,12 @@ ms.topic: conceptual
 
 [Known Issues](../release-notes/known-issues.md)
 
+[Nuget 6.13](../release-notes/NuGet-6.13.md)
+
+[NuGet 6.12](../release-notes/NuGet-6.12.md)
+
+[NuGet 6.11](../release-notes/NuGet-6.11.md)
+
 [NuGet 6.10](../release-notes/NuGet-6.10.md)
 
 [NuGet 6.9](../release-notes/NuGet-6.9.md)