Update Readme.md

See sass/sass-site#1483

Author: nex3

Readme.md

@@ -1,6 +1,12 @@
 LibSass - Sass compiler written in C++
 ======================================
 
+**[LibSass has reached its end of life](https://sass-lang.com/blog/libsass-is-end-of-life).
+It will no longer receive updates of any kind. Users should migrate to
+[Dart Sass](https://sass-lang.com/dart-sass) at their earliest convenience.
+
+---
+
 Currently maintained by Marcel Greter ([@mgreter]) and Michael Mifsud ([@xzyfer])
 Originally created by Aaron Leung ([@akhleung]) and Hampton Catlin ([@hcatlin])