docs: add maintenance warning

README.md

@@ -1,5 +1,9 @@
 # Eth Keyring Controller
 
+> [!WARNING]
+> This package has been merged with [`@metamask/keyring-controller`](https://github.com/MetaMask/core/tree/main/packages/keyring-controller).
+> This repository is in maintenance mode and PRs will no longer be accepted, with the exception of security changes.
+
 A module for managing groups of Ethereum accounts called "Keyrings", defined originally for MetaMask's multiple-account-type feature.
 
 To add new account types to a `KeyringController`, just make sure it follows [The Keyring Class Protocol](./docs/keyring.md).