README: add a note that wire is no longer maintained

Wire was archived by an automatic process because it was not being
maintained. Unfortunately, it's true that there is no one maintaining
wire, and we should make this clear in the README before re-archiving.

Author: findleyr

README.md

@@ -3,6 +3,10 @@
 [![Build Status](https://github.com/google/wire/actions/workflows/tests.yml/badge.svg?branch=main)](https://github.com/google/wire/actions)
 [![godoc](https://godoc.org/github.com/google/wire?status.svg)][godoc]
 
+> [!WARNING]
+> This project is no longer maintained.
+>
+> Those that wish to update or extend wire should do so in a fork.
 
 Wire is a code generation tool that automates connecting components using
 [dependency injection][]. Dependencies between components are represented in