docs: update README.md

Author: ony3000

packages/prettier-plugin-merge/README.md

@@ -60,3 +60,15 @@ export default {
   ],
 };
 ```
+
+## Limitation
+
+All other plugins used with this plugin must be compatible with your version of Prettier.
+
+For example, suppose you have three plugins:
+
+- `prettier-plugin-A`: Only compatible with Prettier v2
+- `prettier-plugin-B`: Only compatible with Prettier v3
+- `prettier-plugin-X`: Compatible with both versions
+
+Prettier v2 users can only configure `prettier-plugin-A` and `prettier-plugin-X`, and Prettier v3 users can only configure `prettier-plugin-B` and `prettier-plugin-X`.