add badges for codecov, release, and HW (#20)

README.md

@@ -1,6 +1,9 @@
 # Mattermost Antivirus Plugin
 
 ![CircleCI branch](https://img.shields.io/circleci/project/github/mattermost/mattermost-plugin-antivirus/master.svg)
+[![Code Coverage](https://img.shields.io/codecov/c/github/mattermost/mattermost-plugin-antivirus/master)](https://codecov.io/gh/mattermost/mattermost-plugin-antivirus)
+[![Release](https://img.shields.io/github/v/release/mattermost/mattermost-plugin-antivirus)](https://github.com/mattermost/mattermost-plugin-antivirus/releases/latest)
+[![HW](https://img.shields.io/github/issues/mattermost/mattermost-plugin-antivirus/Up%20For%20Grabs?color=dark%20green&label=Help%20Wanted)](https://github.com/mattermost/mattermost-plugin-antivirus/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+label%3A%22Up+For+Grabs%22+label%3A%22Help+Wanted%22)
 
 **Maintainer:** [@iomodo](https://github.com/iomodo)
 **Co-Maintainer:** [@hanzei](https://github.com/hanzei)