add codecov badge

Author: Mikhail Podtserkovskiy

README.md

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-# ~~jsonwire-grid~~WebDriverGrid [![Build Status](https://travis-ci.org/qa-dev/jsonwire-grid.svg?branch=master)](https://travis-ci.org/qa-dev/jsonwire-grid) [![Go Report Card](https://goreportcard.com/badge/github.com/qa-dev/jsonwire-grid)](https://goreportcard.com/report/github.com/qa-dev/jsonwire-grid)
+# ~~jsonwire-grid~~WebDriverGrid [![Build Status](https://travis-ci.org/qa-dev/jsonwire-grid.svg?branch=master)](https://travis-ci.org/qa-dev/jsonwire-grid) [![Go Report Card](https://goreportcard.com/badge/github.com/qa-dev/jsonwire-grid)](https://goreportcard.com/report/github.com/qa-dev/jsonwire-grid) [![codecov](https://codecov.io/gh/qa-dev/jsonwire-grid/branch/master/graph/badge.svg)](https://codecov.io/gh/qa-dev/jsonwire-grid)
 This is high-performance scalable implementation of Selenium Grid (hub),
 ###### What is Selenium-Grid?
 >Selenium-Grid allows you run your tests on different machines against different browsers in parallel. That is, running multiple tests at the same time against different machines running different browsers and operating systems. Essentially, Selenium-Grid support distributed test execution. It allows for running your tests in a distributed test execution environment.