docs(readme): add Greenkeeper badge

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README.md

@@ -2,6 +2,8 @@
 [![Coverage Status](https://coveralls.io/repos/telemark/tfk-saksbehandling-skoleskyss-nsb/badge.svg?branch=master&service=github)](https://coveralls.io/github/telemark/tfk-saksbehandling-skoleskyss-nsb?branch=master)
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 # tfk-saksbehandling-skoleskyss-nsb
+
+[![Greenkeeper badge](https://badges.greenkeeper.io/telemark/tfk-saksbehandling-skoleskyss-nsb.svg)](https://greenkeeper.io/)
 Node-module for identifying NSB as transporter.
 
 It uses a combination of zipcodes and schoolids.