Update README with API reference documentation details (#12004)

Author: noahfalk

README.md

@@ -12,6 +12,9 @@ To contribute, see:
 
 - The [.NET Contributor Guide :ledger:](https://learn.microsoft.com/contribute/dotnet/dotnet-contribute) for instructions on procedures we use.
 - Issues labeled [`help wanted` :label:](https://github.com/dotnet/dotnet-api-docs/issues?q=is%3Aopen+is%3Aissue+label%3A%22help+wanted%22+) for ideas.
+- The API reference docs for some assemblies are maintained in the assembly's source code outside this repo. For those assemblies, edits to the XML here are auto-generated and ported, so docs for APIs in those assemblies should be updated by editing the source code comments. For more information, see [here](https://github.com/dotnet/runtime/blob/main/docs/coding-guidelines/adding-api-guidelines.md#documentation).
+
+  Namespaces and types whose docs are maintained in the assembly's source code repo set the [`open_to_public_contributors`](https://github.com/dotnet/dotnet-api-docs/blob/0ddbf94c587e7bdbbadc813a8b58fc4160a47b1f/docfx.json#L164) metadata to `false`. (That metadata disables the Edit button on the published docs.)
 
 ## :bookmark_tabs: Code of conduct