Add build status to README.md

Author: marijnvanwezel

README.md

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 # php-cypher-dsl
 
+![Build status](https://github.com/WikibaseSolutions/php-cypher-dsl/actions/workflows/main.yml/badge.svg)
+
 The `php-cypher-dsl` library provides a way to construct advanced Cypher queries in an object-oriented and type-safe manner.
 
 ## Documentation