Update 'FCNC fitter constants'

FCNC-fitter-constants.md

@@ -1 +1,29 @@
-Welcome to the Wiki.
+The constants are saved in constants.hh and constants.cc. 
+
+
+The constants in `constants.cc` are functions either returning a combination of global constants in `constants.hh`, such as
+
+```
+double CTL_RANGE(){ 
+    return CTL_MAX - CTL_MIN; 
+}
+```
+
+or returns values depending on number of bins, either the Q2 edges of the bins or the S and P observables.
+
+It also contains a function returning a vector of strings with the names of the background parameters depending on the folding and the order of the used polynomial. 
+
+In `constants.hh` global variables are included: 
+* mathematical constants (1 sigma = 0.638, pi, ... )
+* Q2 ranges
+* Angle ranges
+* (LaTeX) names
+* Plotting options
+* PDG masses
+* Mass windows
+* [Particle IDs](http://pdg.lbl.gov/2019/reviews/rpp2019-rev-monte-carlo-numbering.pdf)
+* Angular correction orders
+* Weighting options
+* Parameter values
+* Quite some obsolete constants from the Kspi+ analysis
+