Add API key requirement to Photo Search top level (#1506)

desktop_photo_search/README.md

@@ -1,8 +1,15 @@
 # Photo Search app
 
+This desktop application enables you to search 
+[Unsplash](https://unsplash.com/) for photographs that interest you. 
+To use it, you need to add an **Access Key** from 
+[Unsplash API](https://unsplash.com/developers) to
+`lib/unsplash_access_key.dart`.
+
 This is the Photo Search app, built out with two different widget sets:
 
-  - [Material](material) shows the Photo Search app built with Material widgets
+  - [Material](material) shows the Photo Search app built with [Material widgets][]
   - [Fluent UI](fluent_ui) shows the Photo Search app built with [Fluent UI][] widgets
 
-[Fluent UI]: https://pub.dev/packages/fluent_ui
+[Fluent UI]: https://pub.dev/packages/fluent_ui
+[Material widgets]: https://docs.flutter.dev/development/ui/widgets/material