Add deprecation message to README

Author: nicksnyder

README.md

@@ -1,3 +1,7 @@
+> [!CAUTION]
+>
+> v1 of go-i18n is deprecated and no longer maintained. Please switch to [v2](https://pkg.go.dev/github.com/nicksnyder/go-i18n/v2).
+
 # go-i18n [![Build Status](https://travis-ci.org/nicksnyder/go-i18n.svg?branch=master)](http://travis-ci.org/nicksnyder/go-i18n) [![Report card](https://goreportcard.com/badge/github.com/nicksnyder/go-i18n)](https://goreportcard.com/report/github.com/nicksnyder/go-i18n) [![Sourcegraph](https://sourcegraph.com/github.com/nicksnyder/go-i18n/-/badge.svg)](https://sourcegraph.com/github.com/nicksnyder/go-i18n?badge)
 
 go-i18n is a Go [package](#package-i18n) and a [command](#command-goi18n) that helps you translate Go programs into multiple languages.