docs: add stable version link

Author: typicode

README.md

@@ -1,3 +1,6 @@
+> [!IMPORTANT]
+> Viewing alpha v1 documentation – usable but expect breaking changes. For stable version, see [here](https://github.com/typicode/json-server/tree/v0)
+
 # json-server
 
 [![Node.js CI](https://github.com/typicode/json-server/actions/workflows/node.js.yml/badge.svg)](https://github.com/typicode/json-server/actions/workflows/node.js.yml)