Readme

Created readme file.

README.md

+
+Overview
+--------
+A collection of reference detector geometry for Electron-Ion Collider experiment.
+
+Related useful links
+--------------------
+
+- [Reference_Detector_doc] (https://escalate.readthedocs.io/projects/g4e/en/latest/detector_naming.html)
+- [g4e] (https://gitlab.com/eic/escalate/g4e)
+- [eic_tutorial] (https://argonne_eic.gitlab.io/tutorial/eic_tutorial/part2/adding_detectors/)
+- [DD4hep] (https://github.com/AIDAsoft/DD4hep)
+- [DD4hep_manual] (https://dd4hep.web.cern.ch/dd4hep/usermanuals/DD4hepManual/DD4hepManual.pdf)
+