Add status badge to README

README.md

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 # Hermes
 
+[![Coverage Status](https://coveralls.io/repos/github/HDFGroup/hermes/badge.svg?branch=master)](https://coveralls.io/github/HDFGroup/hermes?branch=master)
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 Hermes is a heterogeneous aware, multi-tiered, dynamic, and distributed I/O buffering system that aims to significantly accelerate I/O performance. See the [official site](http://www.cs.iit.edu/~scs/assets/projects/Hermes/Hermes.html) for more information. For design documents, architecture description, performance data, and individual component design, see the [wiki](https://hermes.page/index.php/Main_Page).
 
 ![Build](https://github.com/HDFGroup/hermes/workflows/GitHub%20Actions/badge.svg)