Update README.md

Author: horike37

README.md

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 [![serverless](http://public.serverless.com/badges/v3.svg)](http://www.serverless.com) [![Build Status](https://travis-ci.org/horike37/serverless-step-functions.svg?branch=master)](https://travis-ci.org/horike37/serverless-step-functions) [![npm version](https://badge.fury.io/js/serverless-step-functions.svg)](https://badge.fury.io/js/serverless-step-functions)  [![Coverage Status](https://coveralls.io/repos/github/horike37/serverless-step-functions/badge.svg?branch=master)](https://coveralls.io/github/horike37/serverless-step-functions?branch=master) [![MIT License](http://img.shields.io/badge/license-MIT-blue.svg?style=flat)](LICENSE)
 # Serverless Step Functions
 Serverless plugin for AWS Step Functions.
+This plugin requires Serverless v1.4.0 or later.
 
 ## Install
 Run `npm install` in your Serverless project.