Add deprecation notice to readme

Signed-off-by: Jayden <jvyden@jvyden.xyz>

README.md

@@ -1,3 +1,6 @@
+# Important: No longer in development
+This project is soon to be entirely replaced by [CosmosFE](https://github.com/LittleBigRefresh/CosmosFE), a rewrite of `refresh-web`. This will stay maintained for the time being as the rewrite matures, but will not receive new features or major upgrades (such as a migration to Angular 17).
+
 # Refresh Website
 
 This is an eternally work-in-progress project intending to build a proper frontend for [Refresh](https://github.com/LittleBigRefresh/Refresh).