added build badge to README (#46)

Author: Claudenw

README.md

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 Aiven connectors for Salesforce on Apache Kafka
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+[![Main checks](https://github.com/Aiven-Open/salesforce-connector-for-apache-kafka/actions/workflows/Merge_check.yml/badge.svg)](https://github.com/Aiven-Open/salesforce-connector-for-apache-kafka/actions/workflows/Merge_check.yml)
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 This is an open source connector developed by Aiven for the community that integrates with Apache Kafka and Salesforce providing both Sink and Source Connectors.
 
 Overview