add badges for codecov, circleci, latest release and HW (#49)

README.md

@@ -1,4 +1,9 @@
-# Welcome Bot Plugin ![CircleCI branch](https://img.shields.io/circleci/project/github/mattermost/mattermost-plugin-welcomebot/master.svg)
+# Welcome Bot Plugin
+
+[![Build Status](https://img.shields.io/circleci/project/github/mattermost/mattermost-plugin-welcomebot/master)](https://circleci.com/gh/mattermost/mattermost-plugin-welcomebot)
+[![Code Coverage](https://img.shields.io/codecov/c/github/mattermost/mattermost-plugin-welcomebot/master)](https://codecov.io/gh/mattermost/mattermost-plugin-welcomebot)
+[![Release](https://img.shields.io/github/v/release/mattermost/mattermost-plugin-welcomebot)](https://github.com/mattermost/mattermost-plugin-welcomebot/releases/latest)
+[![HW](https://img.shields.io/github/issues/mattermost/mattermost-plugin-welcomebot/Up%20For%20Grabs?color=dark%20green&label=Help%20Wanted)](https://github.com/mattermost/mattermost-plugin-welcomebot/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+label%3A%22Up+For+Grabs%22+label%3A%22Help+Wanted%22)
 
 **Maintainer:** [@jfrerich](https://github.com/jfrerich)
 **Co-Maintainer:** [@iomodo](https://github.com/iomodo)