Added readme

README.md

+EIC detectors
+=============
+
+EIC detectors is a collection of detector descriptions and reconstruction 
+parameters need to fully simulate reconstruct a detector system. This 
+repository is for keeping track of different versions  and makes use of the 
+detector libraries built from [`NPDet`](https://eicweb.phy.anl.gov/EIC/NPDet).
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