Add coverrals badge

add coveralls badge #2

Author: SaifJerbi

README.md

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 # SF JAVA UI
 Schema Form Java based library allow developers to define schema and form using field annotations.
 
+[![Coverage Status](https://coveralls.io/repos/github/JsonSchema-JavaUI/sf-java-ui/badge.svg?branch=master)](https://coveralls.io/github/JsonSchema-JavaUI/sf-java-ui?branch=master)
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 The SF Java UI library is a server side extension for the [Json Schema Form](https://github.com/json-schema-form) library. It make your development easier, you will not care about how to define your schema and your screen form definition. Just annotate your fields, and get your screen.
 
 If you use SF Java UI in your project/company please let us know.