Update README.md (#1124)

Author: akeles85

README.md

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 [![codecov](https://codecov.io/gh/hazelcast/hazelcast-cpp-client/branch/master/graph/badge.svg)](https://codecov.io/gh/hazelcast/hazelcast-cpp-client)
 
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 # Async Hazelcast C++ Client
 
 Hazelcast is an open-source distributed in-memory data store and computation platform. It provides a wide variety of distributed data structures and concurrency primitives.