write compilation instruction in Readme; rename CMakeLists.txt

Author: lubynets

PWGHF/D2H/Macros/CMakeLists.txt …2H/Macros/CMakeLists_HFInvMassFitter.txtPWGHF/D2H/Macros/CMakeLists.txt renamed to PWGHF/D2H/Macros/CMakeLists_HFInvMassFitter.txt PWGHF/D2H/Macros/CMakeLists.txt renamed to PWGHF/D2H/Macros/CMakeLists_HFInvMassFitter.txt

@@ -1,13 +1,55 @@
-export ROOT_INCLUDE_PATH=path-to-json-include // if not in O2Physics environment
+# Invariant mass fitter
+Invariant mass fitter is implemented in the class `HFInvMassFitter` (`HFInvMassFitter.cxx/h` files), and its run is executed via `runMassFitter.C` macro.
+Fitter is configured in `config_massfitter.json`.\
+The fitter is **not** a part of O2Physics source code.
 
+## Dependencies
+1. ROOT
+
+2. RapidJSON. Download the header-only (no compilation needed) RapidJSON library, see the link https://rapidjson.org.
+
+## How to run
+### As a ROOT macro
+The `runMassFitter.C` can be compiled as ROOT macro.
+```
+source path-to-root-install/bin/thisroot.sh
+export ROOT_INCLUDE_PATH=$ROOT_INCLUDE_PATH:path-to-json-include
 root -l -x -b -q "HFInvMassFitter.cxx" "runMassFitter.C(\"config_massfitter.json\")"
+```
+If you have O2Physics compilation and enter into its environment there is no need to set environment variables (skip first two lines above).
 
+### As a CMake project
+It is also possible to compile the fitter as a CMake project or insert it into existing one if any.
+Use the `CMakeLists_HFInvMassFitter.txt` (rename it into `CMakeLists.txt` before usage).\
+Compile the fitter with the following steps:
+```
+mkdir build
+cd build
+source path-to-root-install/bin/thisroot.sh
+cmake -DHFFITTER_RAPIDJSON_INCLUDE_DIRS=path-to-json-include ..
+make
+```
+and run the fitter:
+```
+./runMassFitter ../config_massfitter.json
+```
+### Directly from the terminal
+Compile the fitter with the following steps:
+```
+mkdir build
+cd build
+source path-to-root-install/bin/thisroot.sh
 
-Step 1: Generate ROOT dictionary:
+# Generate ROOT dictionary:
 rootcling -f G__HFInvMassFitter.cxx -c ../HFInvMassFitter.h ../HFInvMassFitterLinkDef.h
 
-Step 2: Compile source code:
+# 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:
+# Link the executable:
 g++ -o runMassFitter HFInvMassFitter.o runMassFitter.o G__HFInvMassFitter.o $(root-config --libs)  -lRooFit -lRooFitCore -lEG
+```
+and run the fitter:
+```
+./runMassFitter ../config_massfitter.json
+```