[Physics] Documentation for GEANT 3.21

 GEANT is a framework for simulating the passage of subatomic particles
 through matter, for instance, particle detectors. For maximum flexibility,
 GEANT simulations are performed by linking FORTRAN code supplied by the user
 with the GEANT library, then running the resulting executable.
 .
 This package includes some documentation in text format about GEANT 3.21.