Update README.md

Author: geelen

README.md

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 # CSS Module Loader Core
 > A loader-agnostic CSS Modules implementation, based on PostCSS
 
+[![Build Status](https://travis-ci.org/css-modules/css-modules-loader-core.svg?branch=master)](https://travis-ci.org/css-modules/css-modules-loader-core)
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 ## API
 
 ### core.load( sourceString , sourcePath , pathFetcher ) =><br>&nbsp;&nbsp;Promise({ injectableSource, exportTokens })