add readme deprecation notice

Follow up from the RFC discussions, add a deprecation notice to the README as a first step into deprecating/archiving this repo.

Ref: npm/rfcs#238
Ref: https://github.com/npm/rfcs/blob/cbf076b7a58be9d39d12bd8b91c8cf89612bf8c3/meetings/2020-10-14.md

README.md

@@ -1,3 +1,5 @@
+⚠️ DEPRECATED: This project has been deprecated - **npx** is now part of the **[npm cli](https://github.com/npm/cli)**
+
 [![npm](https://img.shields.io/npm/v/npx.svg)](https://npm.im/npx) [![license](https://img.shields.io/npm/l/npx.svg)](https://npm.im/npx) [![Travis](https://img.shields.io/travis/npm/npx.svg)](https://travis-ci.org/npm/npx) [![AppVeyor](https://ci.appveyor.com/api/projects/status/github/npm/npx?svg=true)](https://ci.appveyor.com/project/npm/npx) [![Coverage Status](https://coveralls.io/repos/github/npm/npx/badge.svg?branch=latest)](https://coveralls.io/github/npm/npx?branch=latest)
 
 # npx(1) -- execute npm package binaries