docs: update README.md

Author: pinpong

CONTRIBUTING.md

@@ -140,6 +140,8 @@ The `package.json` file contains various scripts for common tasks:
 
 When you're sending a pull request:
 
+- **Always open pull requests against the `dev` branch.**
+  Pull requests targeting `main` will be closed.
 - Prefer small pull requests focused on one change.
 - Verify that linters and tests are passing.
 - Review the documentation to make sure it looks good.