[docs] Add Codecov badge

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• README.md
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    [](https://www.npmjs.com/package/slack-message-parser) [](https://github.com/prettier/prettier) [](https://travis-ci.com/pocka/slack-message-parser#) + [](https://codecov.io/gh/pocka/slack-message-parser) Parser library for [Slack message format](https://api.slack.com/docs/message-formatting).