docs(readme): add Greenkeeper badge

greenkeeper[bot] · greenkeeper[bot] · commit ce43173bac7a · 2017-12-11T21:51:26.000Z
diff --git a/README.md b/README.md
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+
+
+[![Greenkeeper badge](https://badges.greenkeeper.io/developit/microbundle.svg)](https://greenkeeper.io/)

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	`3`	`+[![Greenkeeper badge](https://badges.greenkeeper.io/developit/microbundle.svg)](https://greenkeeper.io/)`