docs: add a note about extension activation

README.md

@@ -39,3 +39,7 @@ See <https://vitest.dev/guide/workspace.html> for monorepo support.
 ### How can I use this extension when tests are under a sub directory?
 
 You can use VS Code command `add folder to workspace` to add the sub directory. The extension should work fine.
+
+### Why is the extension not activated?
+
+The extension looks for [Vitest config](https://vitest.dev/config/file.html) or [Vitest workspace config](https://vitest.dev/guide/workspace.html) to establish the connection with Vitest. If you have several config files, it's recommended to combine them in a single [Vitest workspace](https://vitest.dev/guide/workspace.html) for a better CPU performance (only have a single Vitest instance instead of several).