update supported swift versions badge

Author: mattpolzin

README.md

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 # JSONAPI
-[![MIT license](http://img.shields.io/badge/license-MIT-lightgrey.svg)](http://opensource.org/licenses/MIT) [![Swift 5.2](http://img.shields.io/badge/Swift-5.2-blue.svg)](https://swift.org) [![Build Status](https://app.bitrise.io/app/c8295b9589aa401e/status.svg?token=vzcyqWD5bQ4xqQfZsaVzNw&branch=master)](https://app.bitrise.io/app/c8295b9589aa401e)
+[![MIT license](http://img.shields.io/badge/license-MIT-lightgrey.svg)](http://opensource.org/licenses/MIT) [![Swift 5.2+](http://img.shields.io/badge/Swift-5.2/5.3-blue.svg)](https://swift.org) [![Build Status](https://app.bitrise.io/app/c8295b9589aa401e/status.svg?token=vzcyqWD5bQ4xqQfZsaVzNw&branch=master)](https://app.bitrise.io/app/c8295b9589aa401e)
 
 A Swift package for encoding to- and decoding from **JSON API** compliant requests and responses.