init README - describe compilation from the command line

Author: lubynets

PWGHF/D2H/Macros/README.md

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+Step 1: Generate ROOT dictionary:
+rootcling -f G__HFInvMassFitter.cxx -c ../HFInvMassFitter.h ../HFInvMassFitterLinkDef.h
+
+Step 2: Compile source code:
+g++ -fPIC -I$(root-config --incdir) -I path-to-json-include -c ../HFInvMassFitter.cxx ../runMassFitter.C G__HFInvMassFitter.cxx
+
+Step 3: Link the executable:
+g++ -o runMassFitter HFInvMassFitter.o runMassFitter.o G__HFInvMassFitter.o $(root-config --libs)  -lRooFit -lRooFitCore -lEG