doc: add deprecation notice

Author: sedubois

README.md

@@ -1,10 +1,16 @@
-# Mobility Action Text
+# Mobility Action Text (deprecated)
 
 ![Gem Version](https://badge.fury.io/rb/mobility-actiontext.svg)
 ![Build Status](https://github.com/sedubois/mobility-actiontext/workflows/CI/badge.svg)
 
 Translate Rails [Action Text](https://guides.rubyonrails.org/action_text_overview.html) rich text with [Mobility](https://github.com/shioyama/mobility).
 
+> [!WARNING]
+> This project is not maintained.
+
+> [!TIP]
+> Try out vanilla Rails concerns for translating [model attributes](https://github.com/sedubois/mobility-actiontext/issues/59#issuecomment-2670675174) and [rich text](https://github.com/sedubois/mobility-actiontext/issues/59#issuecomment-2672570884).
+
 ## Installation
 
 1. [Install Action Text](https://guides.rubyonrails.org/action_text_overview.html#installation).