fix(readme): add link to the discussion of APIs

Author: deemp

README.md

@@ -16,7 +16,7 @@ That said, you can now use a different set of extensions for `VS Code`/`VSCodium
 - If an extension publisher or name aren't valid Nix identifiers, quote them like `<attrset>."4"."2"`.
 - We have a permission from MS to use a crawler on their API in this case (see the [discussion](https://github.com/NixOS/nixpkgs/issues/208456)). Please, don't abuse this flake!
 - Some previously available extensions may be unavailable in newer versions of this flake.
-  - An extension is missing if it doesn't appear during a particular workflow run in a `VS Code Marketplace` or an `Open VSX` response about the full set of available extensions.
+  - An extension is missing if it doesn't appear during a particular workflow run in a `VS Code Marketplace` or an `Open VSX` response about the full set of available extensions ([discussion](https://github.com/nix-community/nix-vscode-extensions/issues/16#issuecomment-1441025955)).
   - We let missing extensions remain in cache files (see [data/cache](./data/cache)) at most `maxMissingTimes` (specified in [.github/config.yaml](.github/config.yaml)).
 - We don't automatically handle extension packs. You should look up extensions in a pack and explicitly write all necessary extensions.
 - Unfree extensions from `nixpkgs` stay unfree here (see [Special extensions](#special-extensions)).