added readme file for modelica code

modelica/readme.txt

+find in this folder the modelica code for simulation of power consumption in GPS sensors.
+The package contains:
+ - the GPS simulator class (as described in the paper)
+ - inputs (on command and visible satellites) for two different tests scenarios from the paper:
+ 	* TTFFandEphemerisExpirationExperiment
+ 	* visibilityLossExperiment
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