Improve Impeller's docs in the top-level docs folder (flutter#177848)

Author: loic-sharma

docs/engine/impeller/README.md

@@ -1,7 +1,8 @@
-_For user information about Flutter's new rendering backend, check out [Impeller preview](https://docs.flutter.dev/perf/impeller)._
+_For user information about Flutter's new rendering backend, check out [Impeller's public documentation](https://docs.flutter.dev/perf/impeller)._
 
 Team-facing documentation specific to Impeller:
 
+- [Impeller's readme](/engine/src/flutter/impeller/README.md)
 - [Setting up MoltenVK on macOS for Impeller](Setting-up-MoltenVK-on-macOS-for-Impeller.md)
 - [Flutter GPU](Flutter-GPU.md)
-- [Instructions for playing with the Impeller Scene demo|Impeller Scene](Impeller-Scene.md)
+- [Instructions for playing with the Impeller Scene demo](Impeller-Scene.md)