add documentation in README.md

README.md

 # GeoModelTools 
 
-Tools and utilities for GeoModel-based detector description projects
+Tools and utilities for GeoModel-based detector description projects. This includes the CLHEP ExpressionEvaluator 
+(which was moved here to use throughout the project without linking against CLHEP), an XML parser (GeoModelXMLParser) and
+a JSON Parser. The GeoModelTools installation requires the GeoModel suite (GeoModelCore, GeoModelIO), Eigen3, XercesC,
+nlohmann.
+
+## Installation
+
+Detailed installation procedures are available for each and every package, here is a brief and not-exhaustive list of 
+installation procedures for many of them, just bear in mind prepackaged versions are already available (e.g. 
+`brew install eigen` on Mac). Note also that the following procedures install libraries and files under /usr/local, for 
+a local installation use the CMAKE_INSTALL_PREFIX flag, e.g. `cmake -DCMAKE_INSTALL_PREFIX=../install ...`
+
+it is suggested to create a local work directory where all code will be downloaded/built. E.g.
+
+mkdir geomodel_work; cd geomodel_work
+
+### Eigen
+
+git clone https://github.com/eigenteam/eigen-git-mirror.git
+mkdir build_eigen; cd build_eigen
+cmake ../eigen-git-mirror
+sudo make install
+cd ..
+
+### XercesC
+
+from http://xerces.apache.org/xerces-c/download.cgi download e.g. xerces-c-3.2.2.tar.gz
+
+tar zxvf xerces-c-3.2.2.tar.gz
+mkdir build_xercesc; cd build_xercesc
+cmake ../xerces-c-3.2.2
+make -j
+sudo make install
+cd ..
+
+### nlohmann
+
+git clone https://github.com/nlohmann/json.git
+mkdir build_json; cd build_json
+cmake ../json
+sudo make install
+cd ..
+
+### GeoModel tool suite
+
+Note: GeoModelVisualization requires Qt5 and other libraries. Please refer to the gmex documentation
+
+#### Clone and build GeoModelCore from source
+
+git clone https://gitlab.cern.ch/GeoModelDev/GeoModelCore.git
+mkdir build_GeoModelCore; cd build_GeoModelCore
+cmake ../GeoModelCore
+make -j
+sudo make install
+cd ..
+
+#### Clone and build GeoModelIO
+
+git clone https://gitlab.cern.ch/GeoModelDev/GeoModelIO.git
+mkdir build_GeoModelIO; cd build_GeoModelIO
+cmake ../GeoModelIO
+make -j
+sudo make install
+cd ..
+
+#### Clone geomodelvisualization and build gmex
+
+git clone https://gitlab.cern.ch/GeoModelDev/geomodelvisualization.git
+mkdir build_geomodelvisualization; cd build_geomodelvisualization
+cmake ../geomodelvisualization
+make -j
+sudo make install
+cd ..
+
+### Clone and build GeoModelTools
+
+git clone https://gitlab.cern.ch/GeoModelDev/GeoModelTools.git
+mkdir build_GeoModelTools; cd build_GeoModelTools
+cmake ../GeoModelTools
+make -j
+sudo make install
+cd ..