docs: ISS-135490 Add Coveralls badge (#22)

https://app.devrev.ai/devrev/works/ISS-135490

Author: patricijabrecko

README.md

@@ -85,6 +85,8 @@
 
 # Overview
 
+[![Coverage Status](https://coveralls.io/repos/github/devrev/adaas-sdk/badge.svg?branch=main&t=s4Otlm)](https://coveralls.io/github/devrev/adaas-sdk?branch=main)
+
 The ADaaS (Airdrop-as-a-Service) Library for TypeScript helps developers build Snap-ins that integrate with DevRev’s ADaaS platform. This library simplifies the workflow for handling data extraction and loading, event-driven actions, state management, and artifact handling.
 
 It provides features such as: