Updated C# and .NET docs supplement (markdown)

C#-and-.NET-docs-supplement.md

@@ -3,6 +3,13 @@ To save us repeating our complaints about the lack of proper documentation under
 
 Contribute to [the official docs](https://github.com/dotnet/docs) if possible.
 
+## BCL source
+
+The API reference on Microsoft Learn (formerly MSDN) now links to the source in most places, but of course that's for modern .NET.
+- For .NET Framework, use [the Reference Source](https://referencesource.microsoft.com) (or [via GitHub](https://github.com/microsoft/referencesource)).
+- For Mono... [try this](https://github.com/mono/mono/tree/main/mcs/class/corlib) IDK.
+- And for modern .NET where the links are missing, you can search the BCL [on GitHub like so](https://github.com/search?q=repo%3Adotnet%2Fruntime%20path%3Asrc%2Flibraries%2FSystem.Private.CoreLib%2Fsrc%2F**%2F*.cs&type=code).
+
 ## Const (byte/primitive) arrays
 
 Not allowed as either array nor `Span`, despite string literals now effectively having the type `const ReadOnlySpan<char>`,