Add coverage badge

README.md

 This site hosts code for <a href="https://iterm2.com">iTerm2</a>.
+[![Coverage Status](https://coveralls.io/repos/github/gnachman/iTerm2/badge.svg?branch=master)](https://coveralls.io/github/gnachman/iTerm2?branch=master)
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 <a href="https://iterm2.com/bugs">File a bug report here!</a> Issues are on <a href="https://gitlab.com/gnachman/iterm2/issues">Gitlab</a> because Github doesn't support issue attachments.