add description for CEPC prototype.

README.md

 # cepc-offline
 
-CEPC offline software prototype
+CEPC offline software prototype
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+
+## Packages
+
+* Core: Core Software Based on Gaudi
+
+* Simulation: Physics Generator & Detector Simulation
+
+* Reconstruction
+
+* Examples: For new comers
+
+* cepcenv: tools to help developers build packages
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