Add a deprecation message to the readme (#3011)

Closes #2952

README.md

@@ -1,5 +1,11 @@
 # node-sass
 
+**Warning:** [LibSass and Node Sass are deprecated](https://sass-lang.com/blog/libsass-is-deprecated).
+While they will continue to receive maintenance releases indefinitely, there are no
+plans to add additional features or compatibility with any new CSS or Sass features.
+Projects that still use it should move onto
+[Dart Sass](https://sass-lang.com/dart-sass).
+
 ## Node version support policy
 
 1. Supported Node.js versions vary by release, please consult the [releases page](https://github.com/sass/node-sass/releases).