Update README.md

Author: ijsnow

token-highlights/README.md

@@ -4,6 +4,8 @@ A [Sourcegraph extension](https://docs.sourcegraph.com/extensions) for highlight
 
 [**➕ Add to Sourcegraph**](https://sourcegraph.com/extensions/chris/token-highlights)
 
+**Note:** Using this extension on private code in the [Sourcegraph browser extension](https://docs.sourcegraph.com/integration/browser_extension) requires a [self-hosted Sourcegraph instance](https://docs.sourcegraph.com/#quickstart) (because it needs access to the contents of the current file).
+
 Source code on GitHub: https://github.com/sourcegraph/sourcegraph-extension-samples/tree/master/token-highlights
 
 ![demo](https://user-images.githubusercontent.com/1387653/45539771-86065a80-b7bf-11e8-8d44-d9878cbae054.gif)