add README to FLYonPIC Rate Reference implementation

lib/python/picongpu/extra/utils/FLYonPICRateCalculationReference/README.md

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+FLYonPIC Rate Calculation Reference Implementation
+==================================================
+
+Rate- and cross-section calculation in AtomicPhysics(FLYonPIC) are tested by comparison to reference values in FLYonPIC RateCalculation unit test.
+
+The reference values were generated by this python implementation, to regenerate the reference values execute the following:
+
+```bash
+ python calculatorMain.py
+
+```