Add note about unmaintained docs (#2369)

Author: simurai

README.md

@@ -26,6 +26,8 @@
 
 ## Documentation
 
+> :warning: **The documentation of this repo is not maintained anymore**. Please raise any documentation-specific pull requests in [primer.style/design](https://github.com/primer/design/) 
+
 Our documentation site lives at [primer.style/css](https://primer.style/css). You'll be able to find detailed documentation on getting started, all of the components, our theme, our principles, and more.
 
 ## Install