Update README.md

Author: tobias-gp

README.md

@@ -42,13 +42,14 @@ result_future = filter.eval(time=1)
 
 ## Tracker
 
-Oftentimes, multiple objects must be tracked that also require filtering. SimpleFilters implements a simple multi-object tracker for this purpose. 
+Oftentimes, multiple objects must be tracked that also require filtering. SimpleFilters implements a simple multi-object tracker for this purpose. The tracker associates objects by applying minimum weight matching to a distance graph. 
 
 The following properties can be defined: 
 * **distance_threshold**: Maximum distance to match objects - when the threshold is exceeded, a new object will be created 
 * **max_time_to_live**: If an object is not seen, it is still retained for the given number of state updates
 * **time_to_birth**: The number of observations needed until an object is born 
 * **filter_prototype**: A filter that will be cloned for each new appearing object
+* **distance_function**: A lambda (x1, x2) that returns a distance between the two arrays
 
 The **PolynomialFilterStrategy** is especially suitable for tracking, as it can predict the future state of the object according to its reconstructed polynomial: 
 ```