add npm badge

Author: danivek

README.md

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 # json-api-serializer
 [![Build Status](https://travis-ci.org/danivek/json-api-serializer.svg?branch=master)](https://travis-ci.org/danivek/json-api-serializer)
 [![Coverage Status](https://coveralls.io/repos/github/danivek/json-api-serializer/badge.svg?branch=master)](https://coveralls.io/github/danivek/json-api-serializer?branch=master)
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 A Node.js framework agnostic library for serializing your data to [JSON API](http://jsonapi.org/) compliant responses (a specification for building APIs in JSON).