docs(readme): add Greenkeeper badge

Author: greenkeeper[bot]

README.md

@@ -1,5 +1,7 @@
 # Loopback Finite State Machine
 
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 This loopback component provides a finite state machine (powered by https://github.com/vstirbu/fsm-as-promised) for loopback model instances, enabling precise control over when model instance methods may be called.