Add GitHub mirror information to README

README.md

@@ -18,6 +18,11 @@ library is at [go/render](https://git.kirsle.net/go/render) which provides
 the SDL2 and Canvas back-ends.
 (GitHub mirror: [kirsle/render](https://github.com/kirsle/render))
 
+**Notice:** the canonical source repository for this project is at
+[git.kirsle.net/go/ui](https://git.kirsle.net/go/ui) with a mirror available
+on GitHub at [kirsle/ui](https://github.com/kirsle/ui). Issues and pull
+requests are accepted on GitHub.
+
 # Example
 
 ```go