add build badge

Author: baev

README.md

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 # Allure PHPUnit adapter
 
+[![Build](https://github.com/allure-framework/allure-phpunit/actions/workflows/build.yml/badge.svg)](https://github.com/allure-framework/allure-phpunit/actions/workflows/build.yml)
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 This an official PHPUnit adapter for Allure Framework - a flexible, lightweight and multi-language framework for writing self-documenting tests.
 
 ## Table of Contents