Add brief repo description and some health status.

README.md

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 # ROOT Benchmarks
 
+This repository contains a set of relatively small programs (usually based on [gbenchmark](https://github.com/google/benchmark) micro benchmarking infrastructure) built on top of [ROOT](https://github.com/root-project/root). Their primary goal is to provide stable performance metrics which can be monitored over time.
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+# Project Health
+|       |     |
+| :---: |:---:|
+| Linux/OSX [![Build Status](https://travis-ci.org/root-project/rootbench.svg?branch=master)](https://travis-ci.org/root-project/rootbench) | Experimental Benchmark Coverage: [![Coverage Status](https://coveralls.io/repos/github/root-project/rootbench/badge.svg?branch=master)](https://coveralls.io/github/root-project/rootbench?branch=master)  |  
+
+
 ## About
 Collection of benchmarks and performance monitoring applications