Add more info about scalers and GetMetricsin README.md (#372)

* Add more info about scalers and GetMetricsin README.md

* Move new explanation into 'docs' folder

docs/scalers.md

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+## Scalers 
+_This document is in an early stage, contributions and help is appreciated_.
+
+The scalers in KEDA are implementations of a KEDA Go interface called `scaler.go`. The key function in a scaler is `GetMetrics`; it returns a value that represents a current state of an external metric (e.g. length of a queue). Kubernetes HPA will poll `GetMetrics` regularly (as long as there is at least one pod) and compare the returned value to a configured value in the ScaledObject configuration (more about ScaledObjects below). Kubernetes will use the following formula to decide whether to scale the pods up and down:  
+
+`desiredReplicas = ceil[currentReplicas * ( currentMetricValue / desiredMetricValue )]`. 
+
+For more details check [Kubernetes HPA documentation](https://kubernetes.io/docs/tasks/run-application/horizontal-pod-autoscale/).