Update README.md to add codecov badge (#461)

README.md

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 # ![OWASP VulnerableApp](https://raw.githubusercontent.com/SasanLabs/VulnerableApp/master/docs/logos/Coloured/iconColoured.png) OWASP VulnerableApp
 
-![OWASP Incubator](https://img.shields.io/badge/owasp-incubator-blue.svg) ![](https://img.shields.io/github/v/release/SasanLabs/VulnerableApp?style=flat) [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0) ![Java CI with Gradle](https://github.com/SasanLabs/VulnerableApp/workflows/Java%20CI%20with%20Gradle/badge.svg) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg?style=flat-square)](http://makeapullrequest.com) [![Docker Pulls](https://badgen.net/docker/pulls/sasanlabs/owasp-vulnerableapp?icon=docker&label=pulls)](https://hub.docker.com/r/sasanlabs/owasp-vulnerableapp/)
+![OWASP Incubator](https://img.shields.io/badge/owasp-incubator-blue.svg) ![](https://img.shields.io/github/v/release/SasanLabs/VulnerableApp?style=flat) [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://opensource.org/licenses/Apache-2.0) ![Java CI with Gradle](https://github.com/SasanLabs/VulnerableApp/workflows/Java%20CI%20with%20Gradle/badge.svg) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg?style=flat-square)](http://makeapullrequest.com) [![Docker Pulls](https://badgen.net/docker/pulls/sasanlabs/owasp-vulnerableapp?icon=docker&label=pulls)](https://hub.docker.com/r/sasanlabs/owasp-vulnerableapp/) [![codecov](https://codecov.io/gh/SasanLabs/VulnerableApp/graph/badge.svg?token=DTS3PA8WXZ)](https://codecov.io/gh/SasanLabs/VulnerableApp)
 
 As Web Applications are becoming popular these days, there comes a dire need to secure them. Although there are several Vulnerability Scanning Tools, however while developing these tools, developers need to test them. Moreover, they also need to know how well the Vulnerability Scanning tool is performing. As of now, there are little or no such vulnerable applications existing for testing such tools. There are Deliberately Vulnerable Applications existing in the market but they are not written with such an intent and hence lag extensibility, e.g. adding new vulnerabilities is quite difficult. Hence, developers resort to writing their own vulnerable applications, which usually causes productivity loss and the pain of reworking.